In general backlit keyboards coms with black keys with translucent/transparent letters burned into them, allowing the light to shine through the letters but not the keycap itself.
Most other colours would let the backlighting shine through the keycap, diminishing the effect of illuminating the letter on the keycap.
There are a few, a very few, novelty backlit keycaps with symbols on them (skull, flame etc.).
Or there are solid colour translucent keycaps that generally just glow. Or fully transparent keycaps which look kinda cool.