Not everyone sees color exactly the same way, for example some people can see a little into the IR and UV. While the pilots may not be colorblind, the people who repair the displays might be. Situations can also make pilots colorblind, like strong glare coming through a window. It's better to have an unambiguous display that is easy to interpret rather than to rely on something that can be subjective like color. People can only reliably identify a few distinct colors, so if you have 300 kinds of planes and missiles to identify using shades of red and purple doesn't work so well. An ID number next to an icon can handle thousands of kinds of entities. People can tell color #F0479E is different then #F04750 when comparing them side by side, but they probably can't tell you what the exact name of each shade is, and at a glance they might think they are both the same color. So it's not so much colorblindness as it is the limits of human perception. What I call Hunter Green and English Racing Green might look like the same color to you.