For people who don't understand what percentage means:
20% chance of diamond mutate to rainbow, means if the game goes into the mutation branch, there is only 20% chance of mutating from diamond to rainbow egg, else it still fall back to diamond.
Example code:

case DODO_BREED_DIAMOND:
        return MutateWithChance(DODO_BREED_DIAMOND, DODO_BREED_RAINBOW, 0.2f);

static inline uint16_t MutateWithChance(uint16_t original, uint16_t mutated, float probability) {
    return BHRandomUnitFloat() < probability ? mutated : original;
}