diff --git a/static/style.css b/static/style.css
index c910724c6..db423c744 100644
--- a/static/style.css
+++ b/static/style.css
@@ -674,6 +674,81 @@ html[data-palette="dark"] {
     --chrome-fg: #c0ccd8;
     --chrome-muted: #4a5568;
 }
+
+/* ─── Colorblind modes ───────────────────────────────────────────────
+   Override accent/map CSS variables and apply CSS filters to SVG map
+   containers (hardcoded SVG colors can only be adjusted via filter).
+   The filters approximate the color deficiency so that shapes remain
+   distinguishable by lightness and hue shift. */
+
+/* Deuteranopia (green-blind, ~5% of males): shift greens → yellow-blue */
+html[data-cb="deutan"] {
+    --neon: #60a5fa;
+    --neon-dim: rgba(96, 165, 250, 0.38);
+    --neon-faint: rgba(96, 165, 250, 0.12);
+    --accent: #93c5fd;
+    --map-tm: #60a5fa;
+    --map-ics: #fbbf24;
+    --map-asi: #c084fc;
+    --map-gamma: #f97316;
+    --map-special: #94a3b8;
+}
+html[data-cb="deutan"] .zonemap,
+html[data-cb="deutan"] .map-zoom-wrap svg {
+    filter: sepia(0.15) hue-rotate(-15deg) saturate(0.65) brightness(1.08);
+}
+
+/* Protanopia (red-blind, ~1% of males): shift reds → blue-green */
+html[data-cb="protan"] {
+    --neon: #34d399;
+    --neon-dim: rgba(52, 211, 153, 0.38);
+    --neon-faint: rgba(52, 211, 153, 0.12);
+    --accent: #6ee7b7;
+    --map-tm: #34d399;
+    --map-ics: #fbbf24;
+    --map-asi: #60a5fa;
+    --map-gamma: #c084fc;
+    --map-special: #94a3b8;
+}
+html[data-cb="protan"] .zonemap,
+html[data-cb="protan"] .map-zoom-wrap svg {
+    filter: sepia(0.15) hue-rotate(15deg) saturate(0.6) brightness(1.08);
+}
+
+/* Tritanopia (blue-blind, rare): shift blues → yellow-pink */
+html[data-cb="tritan"] {
+    --neon: #fbbf24;
+    --neon-dim: rgba(251, 191, 36, 0.38);
+    --neon-faint: rgba(251, 191, 36, 0.12);
+    --accent: #fcd34d;
+    --map-tm: #fbbf24;
+    --map-ics: #6ee7a0;
+    --map-asi: #f472b6;
+    --map-gamma: #a78bfa;
+    --map-special: #94a3b8;
+}
+html[data-cb="tritan"] .zonemap,
+html[data-cb="tritan"] .map-zoom-wrap svg {
+    filter: sepia(0.2) hue-rotate(40deg) saturate(0.55) brightness(1.05);
+}
+
+/* Achromatopsia (no color vision): full grayscale */
+html[data-cb="achro"] {
+    --neon: #a0aec0;
+    --neon-dim: rgba(160, 174, 192, 0.38);
+    --neon-faint: rgba(160, 174, 192, 0.12);
+    --accent: #cbd5e0;
+    --map-tm: #e2e8f0;
+    --map-ics: #94a3b8;
+    --map-asi: #64748b;
+    --map-gamma: #475569;
+    --map-special: #334155;
+}
+html[data-cb="achro"] .zonemap,
+html[data-cb="achro"] .map-zoom-wrap svg {
+    filter: grayscale(1);
+}
+
 /* interactive SVG wrappers (world map + extension tree) */
 .map-zoom-wrap {
     position: relative;
diff --git a/tools/test_colorblind.py b/tools/test_colorblind.py
new file mode 100644
index 000000000..41202b5be
--- /dev/null
+++ b/tools/test_colorblind.py
@@ -0,0 +1,220 @@
+#!/usr/bin/env python3
+"""Test that the colorblind mode CSS is complete and correct.
+
+Verifies:
+1. All four data-cb modes (deutan, protan, tritan, achro) have CSS rules
+2. Each mode overrides the map color variables (--map-*)
+3. Each mode applies a CSS filter to .zonemap and .map-zoom-wrap svg
+4. The base.html template includes data-cb buttons for all four modes
+5. The initial-load script restores data-cb from localStorage
+6. The palette menu JS handles data-cb clicks and persists to localStorage
+
+Run: python3 tools/test_colorblind.py
+"""
+import os
+import re
+import sys
+
+ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+CSS = os.path.join(ROOT, "static", "style.css")
+TEMPLATE = os.path.join(ROOT, "templates", "base.html")
+
+PASS = 0
+FAIL = 0
+
+def ok(msg):
+    global PASS
+    PASS += 1
+    print(f"  \u2713 {msg}")
+
+def fail(msg):
+    global FAIL
+    FAIL += 1
+    print(f"  \u2717 {msg}")
+
+def section(title):
+    print(f"\n{'='*60}\n{title}\n{'='*60}")
+
+CB_MODES = ["deutan", "protan", "tritan", "achro"]
+MAP_VARS = ["--map-tm", "--map-ics", "--map-asi", "--map-gamma", "--map-special"]
+ACCENT_VARS = ["--neon", "--accent"]
+
+
+# ---------------------------------------------------------------------------
+# Test 1: CSS has rules for all four colorblind modes
+# ---------------------------------------------------------------------------
+section("Test 1: CSS rules exist for all colorblind modes")
+
+with open(CSS) as f:
+    css = f.read()
+
+for mode in CB_MODES:
+    pattern = f'html[data-cb="{mode}"]'
+    if pattern in css:
+        ok(f'html[data-cb="{mode}"] rule found')
+    else:
+        fail(f'html[data-cb="{mode}"] rule NOT found')
+
+
+# ---------------------------------------------------------------------------
+# Test 2: Each mode overrides map color variables
+# ---------------------------------------------------------------------------
+section("Test 2: Map color variables overridden per mode")
+
+for mode in CB_MODES:
+    # Extract the block for this mode
+    pattern = f'html\\[data-cb="{mode}"\\]\\s*\\{{([^}}]+)\\}}'
+    m = re.search(pattern, css)
+    if not m:
+        fail(f'Could not extract block for {mode}')
+        continue
+    block = m.group(1)
+    missing = [v for v in MAP_VARS if v not in block]
+    if missing:
+        fail(f'{mode}: missing map vars: {missing}')
+    else:
+        ok(f'{mode}: all {len(MAP_VARS)} map vars overridden')
+
+
+# ---------------------------------------------------------------------------
+# Test 3: Each mode overrides accent/neon variables
+# ---------------------------------------------------------------------------
+section("Test 3: Accent variables overridden per mode")
+
+for mode in CB_MODES:
+    pattern = f'html\\[data-cb="{mode}"\\]\\s*\\{{([^}}]+)\\}}'
+    m = re.search(pattern, css)
+    if not m:
+        fail(f'Could not extract block for {mode}')
+        continue
+    block = m.group(1)
+    missing = [v for v in ACCENT_VARS if v not in block]
+    if missing:
+        fail(f'{mode}: missing accent vars: {missing}')
+    else:
+        ok(f'{mode}: accent vars overridden ({", ".join(ACCENT_VARS)})')
+
+
+# ---------------------------------------------------------------------------
+# Test 4: Each mode applies CSS filter to map elements
+# ---------------------------------------------------------------------------
+section("Test 4: CSS filter applied to map containers per mode")
+
+filter_patterns = {
+    "deutan": r'html\[data-cb="deutan"\]\s*\.zonemap',
+    "protan": r'html\[data-cb="protan"\]\s*\.zonemap',
+    "tritan": r'html\[data-cb="tritan"\]\s*\.zonemap',
+    "achro":  r'html\[data-cb="achro"\]\s*\.zonemap',
+}
+
+for mode in CB_MODES:
+    pat = filter_patterns[mode]
+    m = re.search(pat, css)
+    if m:
+        # Verify the rule contains a filter property
+        # Look ahead for filter: in the next ~200 chars
+        after = css[m.end():m.end()+300]
+        if 'filter:' in after:
+            ok(f'{mode}: filter rule found for .zonemap')
+        else:
+            fail(f'{mode}: filter rule found but no filter: property')
+    else:
+        fail(f'{mode}: no .zonemap filter rule found')
+
+# Also check .map-zoom-wrap svg selectors
+for mode in CB_MODES:
+    pat = f'html\\[data-cb="{mode}"\\]\\s*\\.map-zoom-wrap\\s*svg'
+    m = re.search(pat, css)
+    if m:
+        ok(f'{mode}: filter rule found for .map-zoom-wrap svg')
+    else:
+        fail(f'{mode}: no .map-zoom-wrap svg filter rule found')
+
+
+# ---------------------------------------------------------------------------
+# Test 5: Template includes data-cb buttons for all modes
+# ---------------------------------------------------------------------------
+section("Test 5: Template has data-cb buttons for all modes")
+
+with open(TEMPLATE) as f:
+    tpl = f.read()
+
+for mode in CB_MODES:
+    if f'data-cb="{mode}"' in tpl:
+        ok(f'data-cb="{mode}" button in template')
+    else:
+        fail(f'data-cb="{mode}" button NOT in template')
+
+
+# ---------------------------------------------------------------------------
+# Test 6: Initial-load script restores data-cb from localStorage
+# ---------------------------------------------------------------------------
+section("Test 6: Initial-load restores data-cb from localStorage")
+
+if 'localStorage.getItem("wiki-cb")' in tpl or "localStorage.getItem('wiki-cb')" in tpl:
+    ok("localStorage read for wiki-cb found in template")
+else:
+    fail("localStorage read for wiki-cb NOT found in template")
+
+if 'setAttribute("data-cb"' in tpl or "setAttribute('data-cb'" in tpl:
+    ok("setAttribute data-cb in initial-load script")
+else:
+    fail("setAttribute data-cb NOT found in template")
+
+
+# ---------------------------------------------------------------------------
+# Test 7: Palette menu JS handles data-cb clicks and persists
+# ---------------------------------------------------------------------------
+section("Test 7: Palette JS handles data-cb clicks and persistence")
+
+if 'querySelectorAll("button[data-cb]"' in tpl or "querySelectorAll('button[data-cb]'" in tpl:
+    ok("querySelectorAll for data-cb buttons found")
+else:
+    fail("querySelectorAll for data-cb buttons NOT found")
+
+if 'localStorage.setItem("wiki-cb"' in tpl or "localStorage.setItem('wiki-cb'" in tpl:
+    ok("localStorage.setItem wiki-cb found")
+else:
+    fail("localStorage.setItem wiki-cb NOT found")
+
+
+# ---------------------------------------------------------------------------
+# Test 8: Achro mode uses grayscale filter (sanity check)
+# ---------------------------------------------------------------------------
+section("Test 8: Achro mode uses grayscale filter")
+
+m = re.search(r'html\[data-cb="achro"\]\s*\.zonemap[^{]*\{([^}]+)\}', css)
+if m:
+    block = m.group(1)
+    if 'grayscale' in block:
+        ok("achro uses grayscale filter")
+    else:
+        fail(f"achro filter does not contain 'grayscale': {block.strip()}")
+else:
+    fail("achro .zonemap rule not found")
+
+
+# ---------------------------------------------------------------------------
+# Test 9: Colorblind modes are independent of palette (no combined selectors)
+# ---------------------------------------------------------------------------
+section("Test 9: Colorblind modes independent of palette")
+
+# Check that there are no combined [data-palette][data-cb] selectors
+combined = re.findall(r'\[data-palette[^\]]*\]\[data-cb', css)
+if combined:
+    fail(f"Found {len(combined)} combined palette+cb selectors (should be independent)")
+else:
+    ok("No combined palette+cb selectors (modes are independent)")
+
+
+# ---------------------------------------------------------------------------
+# Summary
+# ---------------------------------------------------------------------------
+total = PASS + FAIL
+print(f"\n{'='*60}")
+print(f"Results: {PASS}/{total} passed, {FAIL} failed")
+print(f"{'='*60}")
+
+if FAIL > 0:
+    sys.exit(1)
+print("All colorblind mode tests passed.")