I learnt from a book by Steve Allrich called: Oil Painting for the Serious Beginner.
This book changed my life! I was struggling with painting so badly I nearly gave up many times. I read this book and his simple instructions in the chapter about colour transformed my painting.
I still use the limited pallette that he suggests.these colours are what I always use now whether watercolour, oil or acrylic:
Alizarin Crimson Cool Red
Cadmium Red Warm Red
Cadmium Orange
Cadmium Yellow Light Cool Yellow
Ultramarine Blue Warm Blue
Cerulean Blue Cool Blue
Titanium white
Lamp Black
It was incredibly hard for about three months to mix colours using these basic ones but they taught me about colour mixing.
Black and yellow make a nice dull green that is not too vivid
Black and Orange make a good brown
White and Orange is good for flesh tones, with occasionally a touch of cadmium to add a slight pinkness
The two I use mostly apart from White are Ultramarine Blue and Cadmium Orange, use the ultramarine to dull the orange so its not too vivid and similarly use the orange to dull the ultramarine to make the blue not too vivid. When mixed together they make excellent greys, by moving the mix more towards blue the greys can be cooler, or towards orange the greys can be warmer.