6% of males and ~0.4% females have color blindness. A good number of games feature a colorblind mode (Forza 5 for instance). Many don't and have nearly unplayable sections as a result.
When I'm looking for a game, one of my greatest concerns is with color-coded information. If it exists, I need to see it in action, and if it's red-green or any other combination that is a problem, I need to know if I can change it. If I can't change it, I'll usually pass.

Some examples:
RTSs sometimes use red and green icons on minimaps - literally any other combination would be better. Homeworld Remastered's option changes (most but not all of) these to red and blue.
Forza's racing line originally changed from green (gas it!) through yellow (ease off) to red (brake) which is unusable and I'd switch to braking points only. Their option changes the green to teal.
Kingdoms of Amalur uses the common gold, purple, blue, green, and white rarity system for items, and managed to get purple and blue to look different by adding a slight bloom to purple, but screwed up gold and green.
Rocksmith (2011) used lime green, yellow and orange for three of the guitar string which can't be differentiated, and blue and purple for two more... they semi-fixed this for RS2014 by adding a colorblind mode and the results were a lot better, with cyan for lime, a darker orange, and rose for purple. It still wasn't great though, when compared to being able to customize string colors and pick something sensible like white or midtone grey to swap out for other strings.

Generally speaking, I'm used to scouring forums for info on colorblind options, but golly gee it'd be swell to have a database with that information, hint hint!
I'm sure myself and the other 96 million colorblind gamers would be grateful.