Hmm, red-and blue makes sense, given that they are the furthest apart physically.
Obviously accessibility is about more than picking colors, but if a complaint is 'this is a bad color pair' it is interesting to know what is a better pair.
As for contrast beyond hue, does that include saturation? Because keeping luminance constant is an important part of keeping an interface legible.
I'd say a red-blue default with full configurability might be the best approach.
Red and blue are not ”the furthest apart”. Red and cyan, or blue and yellow, are complementary (180° separated on the color wheel). But color perception is more complicated than that.
I believe something like orangish red and bluish green are distinct enough to most people with a red-green deficiency (including myself). The difference in ”blue content” makes them distinguishable.
The actual wave-lengths are the furthest apart. Whilst that is not perceptually relevant, it is relevant to what happens when color-receptors fail.
That is, as long as you have two different color receptors that respond differently to different wave-lengths, those will receive different signals from red and blue light.
The wavelengths that are furthest apart are red and violet. Which perceptually are close together, for red-green blind as well. But what does matter is picking the colors based on the corresponding receptors' peak sensitivity. The L receptors are most sensitive to red, and the S receptors to blue, so in that sense you're right. (Red-green blind or weak people lack the M receptors of have anomalous versions, respectively.)
Obviously accessibility is about more than picking colors, but if a complaint is 'this is a bad color pair' it is interesting to know what is a better pair.
As for contrast beyond hue, does that include saturation? Because keeping luminance constant is an important part of keeping an interface legible.
I'd say a red-blue default with full configurability might be the best approach.