The Temperature of Blue: Which Blues are Warm or Cool?

Blues and tints. Photograph by Lisa Marder

There is much debate over the color temperature of blues. While generally thought of as a “cool” color compared to others, within blues, a blue can either be cool or warm.  I have always considered ultramarine blue to be cool and cerulean and phthalocyanine blue to be warm. However, there are those who would say the reverse. For example, some consider that ultramarine blue is warmer than pthalocyanine blue or cerulean blue because ultramarine blue is closer to violet, which is closer to red, while pthalocyanine and cerulean blue are closer to green, which is opposite from red, and therefore cooler. Even Gamblin Colors states on its website that ‘Ultramarine Blue is so warm that it's almost purple."

While it makes sense in one way that warm blues are those that contain a bit of red, and cool blues are those that contain a bit of green (opposite red and therefore cooler), it does not make sense in another. If blue's bias is towards green, then it must also contain a bit of yellow, since blue and yellow combine to make green. And yellow is indisputably a warm color (at least compared to other colors). Also, if ultramarine blue's bias is purple, that would make it a cooler color since purple is the complement of yellow.

The Wet Canvas website posts a thread on this topic, names omitted, that shows the variety of opinion over cool and warm blues.

Mixing a pure purple from red and blue is difficult because you can't just use any blue or red. In fact, if you're not careful, you might unintentionally be mixing all primaries of the color wheel – red, blue, and yellow. Where does the yellow come from? Yellow comes in the warmer blue, and in the warmer red.  Therefore, it makes sense that the purest purple would come from the cooler red and the cooler blue. When I mix blues and reds to make purple, I find that ultramarine blue and alizarin crimson give me the purest purple. I also find when using ultramarine blue and cerulean blue that ultramarine blue tends to recede and cerulean blue tends to come forward, as is the general rule for cool and warm colors.

Sharon Hicks Fine Art website has an interesting description and discussion regarding blues in her article, WARM OR COOL? Ultramarine Blue vs Thalo Blue.... She says that years ago she learned that ultramarine blue was cool and pthalocyanine (thalo) blue was warm, but she has also more recently come across articles saying the opposite and set out to analyze why that might be. Her interesting analysis is based on the translation of the conversion of the visible light spectrum into a color wheel.  

For resolving this matter, it is best to try your own hand at mixing colors, using different combinations of blues and reds to create the purest purple you can.  For example, try mixing cerulean blue and ultramarine blue with cadmium red or alizarin crimson in different combinations. See the article Color Wheel and Color Mixing for steps to mixing purple and other secondary colors.   However you decide to classify your blues, the important thing is being able to control what they do on the canvas, how they mix with other colors, and how they relate to adjacent colors.

Note: Cobalt blue is generally considered to be primary blue and the most "pure blue."