Generally speaking, the colors, red, yellow and orange are called warm colors and the colors blue, green and violet are called cool colors. But it is important to realise that these terms are just a way of describing the colors and cannot be quantified, like, say the other properties: value (lightness/darkness) and chromaticity (saturation). So, it is not necessary to enter into the questions of which is the warmest or which is the coolest of colors etc. which no one is agreed upon anyway. [In fact, it is easier to think of colors as how much of each of the three hues, red, yellow and blue they contain rather than to think of them as warm or cool; but when conveying or describing them, I think the warm-cool concept is easier]. These terms are not absolute and make real sense only when talked of in a relative way. For e.g Schminke´s Ruby Red is (called) cooler than a Winsor and Newton´s Winsor Red but both reds are warm compared to any blue. You can have a warm and a cool yellow but both yellows are (called) warmer than any blue. Greens can be called warmer than blue because greens have yellow in them.
A color that has more of red (or yellow) is said to warmer than a color which has more of blue. For e.g a Schminke´s Indian Yellow or a Daniel Smith´s New Gamboge both have a tiny amount of red in them and are called warmer than say, a Winsor and Newton Winsor Yellow. A very cool yellow has a tiny amount of blue, which gives the cool yellows a greenish tinge. An example of this would be Schminke´s Auerolin yellow.
The reason why this concept is important is because it is a valuable aid in color mixing. If you want a pure, high chroma mix, then you should mix colors that do not have the opposite color in them (which would naturally dull the resulting mix). A cool red, for e.g has a small amount of blue, therefore it would not be a good idea to add this to a yellow, if you wanted a saturated orange. However, you can do so if you wanted a rusty kind of orange. A warm red plus a warm yellow gives a clear orange. A warm blue (ultramarine blue) and a cool red will give a clear violet. A cool blue (phthalo blue) and a cool yellow will give a clear green.
All this, I have gone into detail in my article Choosing your colors – Color Wheels; I just thought I would clarify it once more before I jump into what I really wanted to write about here: Do warm colors really advance and cool colors recede? ….when we think of reds and oranges we tend to think of high chroma, saturated reds or oranges and we know that objects at a distance start looking bluer and grayer, so it sounds logically right. Warm colors are more attention grabbing, therefore they seem to advance more than cool colors which seem to recede. However, though warm, saturated colors are attention grabbing; if you think of unsaturated warm colors they definitely do not attract more than saturated, cool colors. Compare a mustard yellow or a raw sienna with a medium valued phthalo blue. The mustard yellow seems to recede more than the blue because the blue is so intense. Similarly compare a dull red, like a maroon with a fresh spring green. Definitely, one cannot say that the maroon attracts more than the green. Therefore, it is not strictly true that warm colors advance and cool colors recede. Here´s a study where the results of tests conducted were inconclusive, red patches tended to advance but the same cannot be said for irregularly shaped red objects: The real effect of warm-cool colors: Washington University in St. Louis(2006)
The problem with having such preconceived notions is that it becomes crippling, we limit ourselves even at the planning stage of the painting to fewer possibilities. For a long time now, I had been thinking that it is not warm colors that seem to advance but high- chroma colors that seem to advance, because highly saturated colors are the ones that attract our attention the first. But I have had to revise my opinion on that too. I feel that an artist can make saturated or high chroma colors appear to advance in other ways like with high-detail, high-contrast etc but they do not automatically do so just by virtue of their being high-chroma. Consider, for e.g., this beautiful painting by Frederic Edwin Church, an American landscape painter of the 19th century:
The colors around the sun are both warm and high chroma, yet the area does not advance unnaturally; it does attract the most attention, therefore it is used very effectively as the focal point and the reflection on the water as a path towards it. Definitely, it does look at a good distance beyond the foreground with the track and man walking, the trees catching the light delicately, the flying birds and the waters.
Here´s another painting by him, the River of Light/ Morning in the Tropics (oil on canvas), mesmerising even in its tranquility:
There are no predominantly warm colors in the background here, but it is of a higher chroma than the foreground, yet he has been successful in creating a marvellous amount of depth and distance (which is so necessary in landscape painting) in this work. Two, red-colored birds perched on a palm tree, which catches the light ever so beautifully, a red-flowering bromeliad on the path of brambles leading up to the roots of aged and gnarled trees form the exquisitely detailed foreground, the mid ground is formed by the slightly hazy trees and the white flock of birds just above the waters and beyond that are amorphous shapes for trees at the horizon.
In fact, you can find many paintings of sunrises and sunsets which have warm colors in the background and which work beautifully. Here´s one by Thomas Moran, an American painter from England, called Sunset at Sea; cool, blue-green-gray crashing waves form the foreground and the warm, golden setting sun and swirling clouds and skies form the background:
And finally to wind up, here´s one more of Frederic Edwin Church, Icebergs, which is really a very interesting piece of work:
The foreground is quite cool, blue and bluish white, the shape of the iceberg leads the eye towards the midground which is quite warm, compared to the foreground, a beautiful green-blue which forms the focal point, the iceberg and the background behind that gets warmer(!) they have has touches of orange in it. So, the painting gets warmer as it recedes, yet works beautifully. … It just makes me realize again how important it is to keep the mind free from strict rules.