Computer games are deceptive things. We tend to think they're about killing enemies, talking to people, exploring themes, and so on. This is not the case. What they're about is information. They're about conveying information to the player efficiently so that the player can then make decisions based on that information to advance through the game.
The major mechanic of interaction is still "shooting enemies", but that's modulated. How do you decide which enemies to shoot? I'm going to look at one of the masterpieces of information-based design, Team Fortress 2, to examine the elements of why it's a masterpiece.
As the name suggests, this is a team-based game. You won't be successful alone, but if you coordinate with your team, you'll be successful. To this end, there are two large barriers between teams; the first is colour. Teams are red and blue, which are widely separated colours and which remain easily distinguished even by colourblind players (we'll return to that subject, but for now I'll stay on point). The other method is by popping up names and statuses of teammates when your crosshair swoops over them. Team-mates, incidentally, are impervious to damage from anyone on the same team, and can walk through each other. The pop-ups reassure the player that this person is "us"; opposing players aren't named. They're marked as "other" by the lack of that.
The major flow of information, though, is as a means of giving the opposition information on which guy is the best choice to shoot. The biggest factor in this is what role he's playing; each class has strengths and weaknesses, and some are more critical to a push than others. It's generally a good idea to drop the Medic, for example, since he dramatically increases the effectiveness of anyone close to him. The question is, how do you tell who's playing what class? The answer is design; each class is visually distinct, and the technology in the graphics pipeline is used to made characters pop out from the background and outline themselves. The overall effect of the art design is the feel of an exceptionally violent parody of a Pixar movie, which allows exaggerated proportions and cartoonish emphasis without breaking the feeling of reality.
So we can tell which enemy is capable of what sort of damage, but how do we then refine it? Well, there are persistent effects, such as being set on fire, which can tip the balance. These effects are mostly denoted by a particle effect and a glow. One, however, is very difficult for colourblind players to distinguish; with the colourblindness I have, it's essentially impossible to spot; unless the colourblind mode is turned on. That adds a floating icon over the heads of players suffering this effect. Because of the strange corporate structure of Valve (which is to say its almost complete lack of structure) it's highly likely that this enhancement is the work of one person; whoever he is, I'm glad he works there.
And that's led to the point I originally had in mind when starting this post, which is that visual design is important, and accounting for colourblindness is a very important element in visual design. All our interactions with the world are about information flow, and colourblindness clogs one of the channels of that information. A common thing in the early days of games was the palette swap; to make a more powerful version of a given enemy, use the same base artwork but apply a different colour palette. This worked because it was generally in an environment with a very limited colour gamut; with only 256 colours, even I can generally distinguish colours. Unfortunately, the tradition has survived even though the technical limitations which made it desirable are long gone. One game I've recently been playing a lot is Bastion, which is a gorgeous game, visually, but which has some issues with accessibility. In one area, I'm helpfully told that "greenish-tinted Squirts" will run away; the only trouble is, I can't tell which ones are greenish-tinted. I'm not getting the information I need to deal efficiently with the problem set before me, and so I become frustrated. Granted, this is an optional area, and it's for one of the weapons I don't generally use, but the fact that any area is like this is cause for concern.
So, please, game developers, take accessibility into account. If you add options to make it easier for us, we'll love you for it. If you simply make sure the design is such that issues don't arise in the first place, that's fine too, but be aware that we'll be less aware of your efforts!