The way we see and understand all color is relative.
As a painter I determine what is warm and cool for a select color based upon, not any absolute characteristic of that color, but rather what other colors are interacting and surrounding it. Thus no color is inherently cool or warm, its temperature evaluation is conditional upon what you surround the color with. Admittedly, the extreme saturations of particular hues are nearly color neutral, but in the real world there are no real absolutes, opening up the play of warm and cool relationships to exploit to create volume, shadow, atmosphere and a host of other visual illusions for the artist.
Often I am asked what colors do I use for my skin tones. Invariably I have to ask back 'under what lighting conditions, and with what 'color' flesh?'
There is no standard answer as you will see below. Nearly any color can be used for skin tones, it all depends upon what you surround the figure with!
Search for Mother oil on panel 48" x 36"
Here we have the apparent cool, near death skin tones of a mermaid breathless on the beach. Her cool tones in striking contrast to the warmth of the light and pinkness of the two humans running past. The orange and yellow tones of the rocks, light and seaweed offer a contrasting hue base from the blues and purples of her skin, making it appear very cool and cold.
But with a simple change of background, say making this into a portrait of a woman silhouetted against a blue sky, my choices of color with which I painted these cool skin tones in a near lifeless body take on a fair warm feeling in the context of a bright and blue saturated background. The pinks on the face become more dominant with any tones bordering the neutral hues now appearing to read as 'warm' relative to the cooler background.
Note also how the lighter background in blue has also caused our perception of how 'bright' the face is to change as well...color is not only about relative temperature, but also about value as well!
I have made no adjustments to her face, it is purely a trick your eyes are playing on you.
So the next time you are hesitating with painting a portrait, unsure of how to proceed. Just mix up a pool of diverse color and dive in. Those first marks on the image will never be wrong, as long as you thoughtfully place and develop what goes next to it!
"I can paint you the skin of Venus with mud, provided you let me surround it as I will."