From the email bag: Helen asks, "I have a very basic question for you. I find it so difficult to assign warm and cool to colors,it still confuses me. I am using Golden heavy body paints and need to reorder...is Hansa Yellow Light cool, greenish? Is Hansa Yellow Medium warmish, red? ... Is Ultramarine blue, warm, red?"
Thanks for your question Helen. The easiest way for me to answer this is to say that warm and cool are like the sun and water. The warmer colors are all reds, yellows and oranges. The cooler colors are all blues, violets and purples.
That being said, there are relative warm and cool colors within each broader group. For instance, Ultramarine blue is a very cool blue, while Cerulean Blue is a much warmer blue because it has more yellow in it. On the other hand, Quinacridone Red is a much cooler red than Napthol Red Light because Quinacridone Red has a bluish undertone.
To help understand the nuances of color and whether a color is warm or cool, it's helpful to look at where the color lands on the color wheel. Is it closer to yellow or blue? Take a look at the color wheel above, you will see that magenta is closer to blue than to yellow, therefore it is a cooler red than "red".
It's also helpful to use pigment names rather than the generic names of colors to pin them down on the color wheel. Look at this color wheel from Golden Artist Colors
You can see that by using the pigment names rather than the generic names it's easy to place the colors on the color wheel. Then it's a simple job to answer the question, closer to blue or closer to yellow? That will help you define if a color is cool or warm.
So in answer to your question, Hansa Yellow Light is greener than Hansa Yellow Medium, therefore HYM is warmer. Ultramarine blue is definitely not reddish warm. It's a very cool blue. Cadmium Red Light is very orange so therefore warmer than Cadmium Red Medium which is slightly cooler.
Hope that helps!