There isn't really a color associated with lukewarm. As the diagram in @xiota's answer 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.

There isn't really a color associated with lukewarm. As the diagram in xiota's answer 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.