A lot of games, especially older ones, use lots of colours as part of their gameplay. Older games have things like palette swapping, or colour-coded stuff (like keys).
Just recently it occurred to me that doing so is bad for colourblind users. Check out these doom keys as seen by red-green blindness:
Those keys are supposed to be blue, red and yellow. There's a bit of brightness variation between red and yellow but it's very subtle, especially for the smallest icons at the bottom. If you're still not convinced, here's a pop quiz: what's the colour of these doors, red or yellow?
So have you encountered a situation where something is unclear for colourblind users, and how did you get around it? Is there an open source game that could do with better accessibility for colourblind users?