Ok, not an expert, but I have read up on this some, and here is my understanding. There are warm and cool versions of every color. As for how to tell them apart, this is easier to show than to explain, but I'll give it a go.
- Imagine a color wheel with the three primary colors, red, blue, and yellow. spaced equally.
- In between are all the variations of those primary colors, and also the secondary colors that are made by mixing any two of the primaries.
- Looking at a color's location on the wheel will tell you whether it is warm or cool.
- For instance, think about the reds. The reds that are located between red and yellow are warm reds (because yellow was mixed with them). The reds that are located between red and blue are cool reds (because blue was mixed with them).
- The same kind of thing applies to the secondary colors. In between blue and yellow are all of the greens that can be made by mixing the two. If you were to draw a line to divide the greens down the middle, all of the greens that are on the side closer to blue will be cool, and all the greens on the side closer to yellow will be warm.
Grays, blacks, and browns can be warm or cool, depending on whether warm or cool colors were mixed in to make them. They can also be neutral.
So to tell the difference, you can try to match the color to a color wheel. Another way, let's say if you have blue that you aren't sure about, is to compare it to a blue that is obviously warm, and a blue that is obviously cool. You can usually pick up on which side it is more similar to.
After a while your eyes become trained and it is easier to tell.
Hope that helps a little.