There are commonly two main commercial grades of watercolour paint, one is called "Artists" and the other is called "Students". Although Artists quality paint is more expensive, the pigment is much stronger and it is easier to obtain satisfactory results. Also, Artists quality paints are available in a larger range of colours.
The paint is supplied in either tubes or cakes which are called pans, either whole or half pans.
There are many colours to choose from, but a typical palette of twelve colours would look like this.
Greens can be made from mixing the blues and yellows.
Purples can be made from mixing the blues and reds.
Greys can be made from browns and blues.
Black can be made from a rich mix of Burnt Umber and Ultramarine Blue.
Left click onto the colours to see them full size.