There is already a suggestion for custom colors, but I think 3 more is all the game needs. Currently we have 11, which allows for a byte and 3 more.
I often find myself only needing two or three more colors, for miscellaneous stuff, especially in the CPU levels. I also noticed that the color discrepancy is very high between some of the colors in the "rainbow", whilst in others, it is very small. This leaves room for 3 more, whilst keeping some, if not all, the same.