Learning what the primary colour are and how to use them together is the basis for learning to paint. You understand how to create graduation with in a colour and how to mix the colours.
The three primary colour are Red, Yellow and Blue. There are many different types and shades of Red, Yellow and Blue so when working with them you will get a different result depending on what colour variations you are using.
I think the first step would be to put out your primary colours on to a page. You could put them in a circle or draw out a small rectangle and paint the colours into that space
The next step would be to create a graduation reference based on the primary colour you are using. To do this use a little water on your paint brush and go to your first colour for instance Blue. Then paint the blue onto your paint. Next step is to add more water onto the paintbrush and go back to the blue. Move the paintbrush down down and as you go down the colour will start to fade into a light colour.
Video about the Primary, Secondary, Tertiary colours
It shows how to mix the colour and about creating different shades