There is a lot of debate about the temperature of the blues.One shade up from violet, within the range of blues, a blue can either be cool or warm.The relative temperature of a particular blue is open to interpretation.Some artists say that cerulean and phthalocyanine blue are cooler than ultramarine blue.
The temperature of a color is a relative measure, it can only be “cooler” or “warmer” than another color.It is a matter of opinion.
There is no argument about the location of the color wheel.On the traditional color wheel, ultramarine blue is closer to violet than phthalocyanine and cerulean blues.Green and violet complement each other on the color wheel, so you can decide if ultramarine is cooler.
If blue’s bias is towards green, then it must also contain a bit of yellow, since blue and yellow combine to make green.Yellow is a warm color, at least compared to other colors.Ultramarine blue is a cooler color since purple and yellow complement each other.
The issue is not settled.Many established artists have opinions of their own.
Her article asked if it was warm or cool.Years ago she was taught that ultramarine blue was cool and phthalocyanine was warm, but she has recently come across articles saying the opposite.The conversion of the visible light spectrum into a new color wheel is what she believes is the reason for that.The violetish ultramarine is cooler than the greenish thalo or cerulean blues on the cool end of the color spectrum.