Choose one yellow, one blue, and one red. Use those for your entire painting, mixing all your other colors from those three, and PRESTO! You will have a harmonious painting. You will see an almost instant improvement in the color harmony in your artwork. Pre-mixed colors may be convenient, but they look out of place and the entire piece will not come together as a whole. Just something to think about.
Here is the palette that I use. Cadmium Yellow Pale, Ultramarine Blue, and Alizarin Crimson. I am not endorsing Windsor Newton. That is just the brand I use.
Notice that I have one yellow, one blue, and one red. I also have white and Burnt Umber. Notice that I have no black. That is not a mistake. I find that another great way to keep my colors consistently harmonious is to mix my own black. Pre-mixed black is very flat and boring. I use Ultramarine Blue, and add a little red, a little yellow, and a little Burnt Umber to create a colorful, rich black. It is darker than black from a tube and far more appealing.