Warm and cool refer to the hue, not so much the chroma. Warm colors are generally red, yellow and orange. And cool colors are green, blues and purple. Of course there are warmer greens and cooler greens so this term warm/cool is more of a way to compare colors to one another. And it is okay to combine warm and cool colors. A common color scheme is to place complimentary colors together which come from opposite sides of the color wheel. So when you do this you always get a warm and cool.