Lukewarm isn't a good candidate for an antonym, because it describes a specific temperature. Not an exact specific temperature, one can't mark "lukewarm" on a thermometer, but consider being presented with two bowls of soup and asked which is "more lukewarm". Either neither is, one is and the other isn't, or both are, lukewarm. In the last case, how do you decide which is more so? Is it the slightly warmer one, or the slightly cooler one?
It's like with cooking steak: you could say well-done is the antonym of rare, I'd agree[†] with that as a premise. But medium doesn't have an antonym, and it makes less sense to describe the antonym of medium-rare as medium-well, they aren't opposites.
[†] If you would prefer "blue" or even "raw" for maximum contrast, that's fine by me, both cool and cold are antonyms[‡] of warm, same principle.