My Color Palette
This is a section on my standard color palette so that you can see what I'm talking about when I refer to the colors I'm using.
If you're just buying your first paints, read my section on the essential colors you need to get. It's better to start out with only a few colors, so you'll learn good mixing skills. As you add to your palette, you'll find that you prefer some shades over others. My preferred manufacturer, Golden, currently makes about 100 colors. I use about 20 in my standard palette.
The names of paint colors vary between brands and types of paint, but the base pigments are the same. All professional quality paints list the base pigments they use, so if you migrate brands you know what you're working with.
Notice that these paint swatches show the transparency of the paint in addition to the color. transparent paints show the brush strokes more than opaque paints. You can notice this especially in the two white shades I use.
Blues
My favorite color is Ultramarine Blue (my default color for blue sky) which is fairly opaque and can be warmed up with Pthalo Blue. Pthalo Blue can be difficult to use at first because of the high color strength, but once you get used to it and learn how to thin it down, it is quite nice for water.
Greens
Sap Green is one of Golden's historical colors, a synthetic version of a paint originally made from the sap of Buckthorn berries. I like it for painting leaves because it has a nice natural hue. Green Gold and Pthalo Green are very versatile transparent greens, but they both need a lot of mixing with other colors to produce realistic plant shades.
Yellows and Oranges
I originally just used Primary Yellow and Yellow Ochre, but they are both fairly opaque and tend to look chalky when thinned out in glazes. I added the Nickel Azo Yellow for tinting other mixes and bringing in transparent warm layers, when I want the underlayer to show through without being lightened.
Reds and Browns
Burnt Umber is a very versatile color, but I find it tends to range a lot in lightness when you go between brands, and it's not as transparent as I'd like. When torn between choosing two shades for your palette, I usually recommend choosing the more transparent shade, since you can always mix in some Titanium White or Carbon Black if you want opaque effects. I'm always on the hunt for a more transparent brown.
Pinks and Purples
I use Quinacridone Magenta and Dioxazine Purple in nearly every portrait I paint. Violet Oxide is much more opaque and not as versatile, but it's a wonderful color for painting chocolate labs, so I use it pretty often.
Blacks and Whites
My favorite discovery of last year had to be Zinc White. The transparency allows you to lighten areas with a lot of control, which is very useful when making adjustments in portraits or adding shine on tricky subjects like eyes. Bone Black is the black equivalent of Zinc White; though not quite as versatile, it's still useful to have a black that can darken without covering.
In addition to these colors, I use some metallics and interference colors for special effects.