Last week I mentioned that I wanted to make the sky cooler in one of my demonstration paintings and therefore I would use the color Cerulean Blue, a bluish green color. A student said that he thought that Cobalt Blue was cooler.
There are many color theories and it depends on which one you choose to use. The one that I use and teach is the CIE XYZ Lab color model. That theory states that Red-Orange is the warmest color and Blue-Green is the coolest color. Therefore Cerulean Blue would be cooler that Cobalt Blue.
Which of the many color theories you use is up to the individual artist.
Just pick one and be consistent when painting.