I'm not color blind but generally avoid reliance solely on color coding. Best visual designs are those that work equally well in grayscale and in color. As Calinou already hinted, you can use other visual design elements to differentiate objects: shape, texture/pattern, size...
On top of that you can numerate your keys (A, B, C, 1, 2, 3). You can name them using names that relate to your game narrative or level theme, and then reflect that name in the design.
For best results employ several of these principles at once. Color coding can then be cherry on top.
Keys don't have to be literal keys either. They can really be anything, from abstract numbered geometrical symbols to characters or pet animals.
One example. Let's say you want actual keys. Your key icons can then be small keys with large keychains that are animal-themed. Each keychain is an animal icon with a distinct animal shape, different two-color scheme and a name shown whey you pick it up or encounter corresponding door (e.g. fox key, lizard key, scorpion key)
Possibilities are endless :)
