At the salon, I hear all the time that in certain lights their hair looks warmer and in others it looks cooler. Well, it all has to do with how your hair is illuminated, some of the light source's wavelengths are absorbed and some are reflected. The reflected wavelengths create what we perceive as color. for example if no light is reflected (or if there is no illumination), the object or hair looks black. So in direct sunlight, see below, it has white light which reflects, absorbs, or transmits all the colors of the rainbow. This is when you will visually see all the colors that you have in your hair. (normally on the warmer side).
I wanted to demonstrate how lighting effects the way we see color. I colored my hair today, and took these photos within minutes of each other. Of course, all the photos are untouched and unedited so you can see the true color with all the light sources.
Natural lighting outside
Indoor window lighting