I mentioned a couple of limited palette approaches in this blog before. They are very helpful in keeping color harmonies under control. Because you only have a few tube colors with which to work, you're essentially using the same colors in different amounts to arrive at every new mixture. It's almost impossible not to achieve harmony.
There are many ways to set up a limited palette, obviously. This one is a variation of the earthtone primaries set up. Usually when I do a primaries palette with just earth tones, I use Transparent Oxide Red for my red, Ivory Black for my blue, and Yellow Ochre for my yellow, and White. It's a very muted palette, guaranteeing that your saturation doesn't get out of control even if you tried.
But it does have a very earthy, pre-impressionist look to it. Which isn't a bad thing, but if you're itching for more color, you'll want to replace one or more of the primaries with a more saturated version. For example, if you replace Transparent Oxide Red with a Cad, you end up with the Zorn palette. (He used Vermillion, but the same idea)
Or, you can keep your muted primaries and add a more saturated color just where you want to punch things up a bit, and that's what I did with this sketch. I used the TOR-Ivory Black-Yellow Ochre-White palette to begin the painting and took it as far as I could, then added Ultramarine to my palette, in order to get the blue in the jeans and the little bit brighter greens in the backgrounds, plus the violet in the sofa.
The warm tones didn't need to be pushed, because all the cool tones surrounding them made the small amount of warm tones seem warmer than they actually were.
Small variations go a long way. And keeping my palette limited keeps me from getting confused and overwhelmed by having too many choices.