New answers tagged graphics
Re-designing the graphics is a GOOD idea, from a UX standpoint, though it may be problematic if you're trying to bolt color-blind support onto an existing game. The important thing is going to be making the keys somehow LOOK like they match the doors. I think the easiest way is to do something with the orientation of the key, and the orientation of the lock ...
Having a distinct outline helps a lot with icons. You could change the orientation of the key to represent the 4 different colours (i.e. change colour and orientation). Something like this should be distinct enough:
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