"Readable" is a somewhat subjective measure, dependent on things like monitor settings, gender (most colour deficiency is in Males) and age (the amount of light going through the cornea of a 60 year old is under 25% of what used to go through at age 18).
You could take a slightly "brute force" approach by preserving the hue, forcing saturation to 1.0 and brightness to 0.5.
(As I recall from an article I read a couple of years ago, the amount of light going through the cornea of a 60 year old is under 25% of what used to go through at age 18. According to an article I found here, the figure is worse still - light transmission ranges from a high of 16% (1/6th) and a low of 6% (1/16th) depending on conditions.)