Around 8% of men and 0.5% of women see color differently — mostly along the red-green axis. Feed in a chart, a UI screenshot, or a palette PNG, and see whether your color coding survives the translation.
Look at the deuteranopia panel — the most common type — and ask one question: can you still tell apart the things that need telling apart? Chart series, error-vs-success states, subway lines. If two of them merged, the fix usually isn't new colors: it's adding a second channel — lightness difference (which the grayscale panel tests), labels, patterns, or icons. A palette that survives grayscale survives everything. Build one from a photo you love with the palette extractor, then confirm text pairs with the contrast checker.