There isn't really a color associated with lukewarm. As the diagram in [xiota's answer](https://ux.stackexchange.com/a/126227/7202) shows, the color association humans have (red = warm, blue = cold) are even the opposite to what you'd expect from a physics point of view.

- Since 'lukewarm' is the neutral option, you could go with a neutral color: **gray**. Do make the button's style different from disabled buttons, if you have any. 
- Since 'lukewarm' is the middle option, you could go with the color in between red and blue: **purple**.
- Since 'lukewarm' is presented in the middle, users will know it's the neutral option in between warm and cold **regardless of the color of the button**. Of course, its color shouldn't lean too much to one of the other options; I wouldn't choose another shade of blue or red.