I really like the idea! I start from checking different options in the menu, so I checked the controls before starting, but I kept forgetting which key corresponds to which color. Maybe it would be worth adding color indicators on the in-game UI with? It could show two things:
1. Which key activates which color
2. If you have that color available
The second point feels important for me, as I didn't get it at the beginning why I can't do anything after hitting S. Later I kept forgetting that I can use white. :D
Another sugestion that could make your game more accessible for color-blinded people is to add textures with patterns connected with each color. I think about something like on screenshots of this asset for Unity.