Colour blindness test

This is a hue arrangement rather than an Ishihara plate, and that is a deliberate and fairly important choice. Ishihara plates — the dotted circles with a number hidden in them — depend on precise printed pigments under controlled lighting. Reproduced on an uncalibrated screen at whatever brightness you happen to have, they test your monitor at least as much as your eyes, which is why an online "Ishihara test" can tell a person with normal vision that they are colour blind and vice versa.

Arranging hues by similarity is the basis of the Farnsworth–Munsell tests, and it degrades more gracefully. The score is the standard total error: for each cap, how far its position is from its two neighbours in the true sequence. A perfect arrangement scores zero, and because the caps form a ring, starting at a different point or going the other way round is not an error — only genuine mis-ordering counts.

The two end caps are fixed as anchors. Without them there is no reference for which direction the ring runs, and a perfectly-arranged ring in reverse would be indistinguishable from a random one.

What this cannot do is tell you which type of colour vision deficiency you might have. Distinguishing protanopia from deuteranopia needs specific confusion axes tested under known conditions — an anomaloscope, in a clinic. What an arrangement score gives you is a rough signal of whether your discrimination is in the usual range. If it is not, turn the brightness up and try again before concluding anything, and then see an optometrist.