Recently, I was flipping through the mail order catalog of a national arts supply chain. Amongst all the other nice products available was a set of 5 oil paints labeled "Beginners Set" or something like it. It appeared to consist entirely of hue or mixture colors based on modern synthetics, like Hansa Yellow, Napthol Red, Pthalo Blue, etc. The striking thing about this particular selection was the absence of ANY of the earth colors. Granted, not all beginners will use such palette, but I think that novice painters tend towards brighter and stronger colors. Given such a palette, it's easy to see why so many beginners have disappointing results; acid green trees, jaundice yellow flesh tones, electric blue skies, and all the like.
Don't get me wrong; I LOVE many of these contemporary synthetics, and I strongly believe that a good painter should be able to use any paints whatsoever. But, at the end of the day, I would have to estimate that approximately 75% to 85% of the paint that ends up on my canvas is earth. These humble but essential pigments are exactly what they sound like; dirts and rusts. Specifically, ochres, siennas, umbers, oxides, and a few exotic semi-precious stones. They are available in a very wide range, and some manufactures (in particular Williamsburg) offer earths from very specific geographical locales for the true connoiseur.
The distinguishing feature of the earth tones is their low color saturation and fairly narrow band of actual color hues, in other words, they tend to be very gentle and muted colors. Because of this, they are absolutely ideal for reducing the chroma of more intense colors and providing a strongly unified color harmony. No matter which earth tones are used, in fact, given enough earth paint on a canvas, it would be difficult NOT to achieve some degree of unity. They also provide a more neutral ground on which to advance strong colors in a more dramatic fashion. Given that fact that it's basically dirt, it automatically takes on some of the color schemes found in Nature.
In my own palettes, I generally try to pair contemporary/synthetic colors with an earth analog. For instance, cadmium red with burnt sienna, cadmium yellow with yellow ochre, viridian green with terra verte, etc (blue is tricky - there is a blue ochre, but it's very rare and I've never had the chance to use it). In practice, this never works out that cleanly, but I've found it to be a pretty good rule of thumb.
So, beginners, ditch the glowing technicolors, and fill up your paintings with dirt. You'll be glad you did.