Two marks, one grey
Every rule this wall has about colour asks one question: is
there enough contrast between this ink and the ground under it. That is
a ratio of brightnesses, and it is the right question — right up to the moment
a figure draws two marks and asks you to tell them apart from each other.
Nothing here had ever asked that. So from its first figures onward this wall
drew its two main series colours, an amber and a green, at
— in brightness: the same shade of grey, differing
only in hue. For most readers that is two obviously different colours. For the
complete form of red-green colour blindness — about one man in fifty, and
what the patches below model — it is one colour; for the commoner milder
forms, about one man in twelve, much of the difference goes with it. For
anybody at all it is one colour in print.
The wall's inks
The five colours this wall draws its series in, and the same five as three kinds of colour blindness leave them. Nothing is exaggerated for effect: the patches are the output of a published model of each condition, applied to the exact colours in this page's stylesheet.
What that does to a figure
Two curves, drawn the way this wall draws them, in the amber and the green above. Switch the reader and watch the legend stop working — because the legend is a row of coloured swatches, which is the same cue a second time rather than a second cue. A key that tells you the amber one is the first series is no use to somebody who cannot pick out which one is amber.
Brightness is the part that survives
Losing a cone costs you a direction in colour, not the whole of it. What is left is brightness, and one axis of hue — blue against yellow for the two red-green conditions, red against green for the rare third. So two marks are findable when they differ in brightness, whatever happens to their hue: this wall's green tick and red cross are the textbook confusion and have never been a problem here, because one is half as bright as the other.
Here is every ink on one brightness axis. The dotted line is where the amber sits. The old green sat on that line; the one that replaced it does not, and that is the entire repair.
—
Every pair, and what each reader gets
Two numbers per pair. The first is the brightness ratio — 1.00 means the two marks are one shade of grey. The second is how far apart the colours are, in the units a colour-difference formula gives, for the reader who has the hardest time with that pair. A mark is lost only when both go: same brightness, and the hue that separated them is the hue that reader has not got.
What this page checks about itself
Three of these are the instrument pointed at answers that were known before it existed: a published set of colour-difference pairs, and the two facts that name the conditions — a deuteranope loses red from green and keeps blue from yellow, and a tritanope does the reverse.