The BBC reports that colorblind gamers could be at a very significant disadvantage in online gaming. Colorblindness affects 5% of men and .5% of women, which deficiencies including making it hard to tell apart red/green, blue/yellow, and in rare cases, total colorblindness.
Colors are important in a lot of games, especially team games where everyone on your side is flagged with a specific color. Like Team Fortress using Red and Blue, and as the article states, Call of Duty using Red and Green tags, the most common form of colorblindness if I recall.
I personally know someone on Phantasy Star Online that cannot tell Blue and Yellow apart because she has tritanopia, which puts her (or him, I don’t know) at a disadvantage in one area of the game where there are three incredibly common monsters that are palette swaps of each other, each with its own elemental weakness. PSO being a difficult game means that the Ruins is a nightmare for her. She eventually reskinned them to colors she could tell apart but that took a lot of time and effort along with having to learn how to skin things in the first place.
In a lot of these cases, the color issue could be solved in an options menu with things like “Player team color, enemy 1 team color, enemy 2 team color” and so on, overriding the default settings and allowing people to use what colors they wish so they don’t have to be at a downside because of a situational disability. Black Ops has an option that does this thankfully.
There is no way to appease every colorblind person, but it’s a common enough problem to be at least given some consideration. Clientside, people can skin things, but skin modification of any kind in a game is often considered highly unethical and cheating for multiple reasons. As I had said, players being able to customize the interface and display of the game where colors are important would make people a lot happier and be an option for everyone.