How to Choose Colours Procedurally (Algorithms) » devmag.org.za: "In each row, the hue differs by 5%. Notice that certain colours cannot be distinguished. Also notice that it is different depending on the brightness. Lighter blues is more distinguishable than darker blues, while darker magentas are more distinguishable than lighter ones.
This fact becomes important when you try to select colours that look “equally” far apart; in general, it cannot be done without using a perception-based colour space (such as LAB colours), or making appropriate adjustments in other colour spaces."