In the advanced edition, it makes clear how much benefit your characters gain from levels of skill with hand-to-hand combat. Specifically, green means +10% damage and yellow means +20% damage. However, Engi deal 50% damage to begin with. Does a green level of skill mean they deal 60% damage (additive) or 55% damage (multiplicative).
This question also addresses Mantis who also have their base damage adjusted. I'd imagine they'd work identically but whether the bonus is additive (210% green/220% yellow) or multiplicative (220% green/240% yellow) is not known to me.
This question is agnostic between advanced and standard editions unless they unexpectedly vary.
Edit: This matters more than I thought. Mantis actually deal +50% damage, not +100% damage as listed in the game. I don't know how I missed this for so long. The reason this matters is that a Rock with yellow combat will deal 120% damage with 150 HP (combat rating: 180) while a Mantis with yellow combat will either deal 170% damage with 100 HP (combat rating: 170) or 180% damage with 100 HP (combat rating: 180).
Thus, a rock becomes the ultimate fighter even ignoring the fire immunity the additive case, whereas they are tied in the multiplicative case. I'm defining combat rating as HP * combat damage%. A higher combat rating means you'll always win in a one on one fight with no other advantages on either side.