It's confusing to look at nature and determine whether the visual temperature is warm or cool. Yet master artist Richard Schmid is adamant (and I agree) that the temperature of the light is our most important harmonizing element because it effects all the colors illuminated by it.
But even Schmid admits that determining what we see as warm or cool can be tricky. Here are his own words:
"Generally speaking (and only generally), sunlight is warm. Consequently, the more overcast the sky, the cooler your light will be. Mother Nature is very tricky though. She can throw you a curve when you least expect it. Trust your eye always. " (Taken from Schmid's website FAQ page)
Now, trusting our eyes look at this:
In our discussion about value I emphasized that we see because of light. A simple concept, but the heart of every decision we make when painting. Now we go one step further: every light source has a color, therefore a temperature--it is either warm, cool or neither. Forget about the "neither" and assume as an artist you're dealing with either warm or cool.
Both photos above show the same batch of lemons, yet the photo on the left is in warm light and the one on the right in cooler light. Visual temperatures are cooler when they are bluer, warmer when they are redder or yellower.
Now look at this:
The ball is blue (well, of course it is !) Blue is the coolest of colors, but notice where a warmer light hits the blue ball most directly, the color appears warmer. That same blue appears cooler as it moves into shadow. I've isolated these with samples across the top to make it easier to see. (You might have recognized that I used this same illustration in an Empty Easel tutorial about color.)
The reverse happens when a color is illuminated by a cool light.
Let's look at our lemons again--the ones in cool light. Now let's zoom into a shadowed area and sample it. The results are in the rectangle on the upper right edge.
Joila! The shadowed areas are warmer (more orange) than those more directly in light.
Perhaps this simple principle is the most important one to remember when dealing with temperature and color. Our natural tendancy--indeed, some have taught--is to always make shadows cooler, but in painting it's best not to live by rules other than always let your eye tell you what your looking at.
As a general rule of thumb, though, if you shift your eyes between light and shadow of an area, you WILL see a difference in temperature between the two. One will be cooler, the other will be warmer. Most likely, if the shadow is warmer then the light source is cool. If the shadow is cooler, the light source is warm.
Oddly, if you direct your painting choices by keeping this in mind, the painting should feel in tune or in harmony with itself.
And so, the conclusion we always come to is teach yourself how to see, then trust your eyes.