Our eye can have 3 color cones:
red, green & blue—and maybe yellow—
but our cones can fail us.
For example, our red cone can
not respond as much, Our green cone
could [theoretically] vanish, and so on.
Some people see red, yellow & blue (Deuteranomaly),
yellow & blue (Protanopia,) red & blue (Tritanopia),
or even grayscale (Acromatopsia).
Even though these seem detrimental to design,
you can design around them with clever palletes.
For example: You can use light yellow/orange and dark blue
to allow Protans and Acromats to see it better.
000 FFF F00 00F 800 008 F88 88F 888 BBB
000 FFF F00 FF0 800 880 F88 FF8 888 BBB
000 FFF 0F0 00F 080 008 8F8 88F 888 BBB
000 FFF 0F0 FF0 080 880 8F8 FF8 888 BBB
000 FFF FF0 00F 880 008 FF8 88F 888 BBB