Color Temperature: A Pigment's Tell and a little bit about Blue

The other day I was wondering what the difference was between Ultramarine Blue and French Ultramarine Blue (because that is something normal people wonder about), and I learned something new about color temperature.

99% of the time the color temperature of a paint is easy to tell, but then there is that 1%. For me, it is with blues. I forget which is which and need to look them up. Before, in my head I classified colors into how close they were to blue or orange, blue being the coolest and orange being the warmest.

The problem that I ran into with trying to decide the temperature of Ultramarine and French Ultramarine, beyond that they are both blue and therefore cooler than most colors, is that they reside on opposite sides of "true blue." Does this make the both of them warm, since they are moving away from blue? But then how does that work with cool blues? I was confused and took to the internets.

I found a lovely little post from the American Society of Botanical Artists that explained the difference between warm and cool in a way that should have been obvious.

Most pigments will either have a green or red bias, not an orange or blue bias. Those with a green bias are cool while those with a red bias are warm. Thinking of warm and cool in this way simplifies, at least for me, color temperature.

Ultramarine has a slight green bias and therefore is cool. French Ultramarine leans towards violet with a slight red bias and therefore is warm. Ta da! Mystery solved! You can read more about their differences here.

You can also read about why color temperature is important for accurate lights and shadows here and check out my main Tips and Techniques page for more information about painting.