What is the color associated with lukewarm?

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.

1. What color represents the warmest temperatures?
2. What are the cool colors?

What color represents the warmest temperatures?

While every degree represents a new hue, you can use broader terms to describe a color family; red, orange, yellow, etc. Think of the color wheel as a clock where every hour marks a new color family. Absolutely warm and cool colors can be found at 0 (red – the warmest color) and 180 (cyan – the coolest color) degrees.

What are the cool colors?

Cool colors include green, blue, and purple, and variations of those three colors. Blue is the only primary color within the cool spectrum. Greens take on some of the attributes of yellow, and purple takes on some of the attributes of red. They are often more subdued than warm colors.