Small pea sized amount of each of each colour your pallet - you don't need very much paint all:

 

Yellow

Red/Pink

Blue (Cobalt or Ultra Marine Blue)

 

Plus any convenience colours you’d like

 

Remember -     Red, blue and yellow are the primaries

            Blue and Yellow = Green

            Red and blue = purple

            Yellow and red/pink = orange

 

All primaries together make grey, or ultramarine blue plus burnt sienna

Ultramarine blue and burnt umber make a lovely rich ‘almost’ black colour

            

There is no white in watercolour,  the white comes from the paper.  

To lighten a colour add more water

 

To tone down (desaturate) a colour add it’s opposite on the colour wheel.  For example adding a tiny bit of red to green will give you a lovely sage green colour

 

Yellow is the weakest colour.  When using yellow to make a colour always start with yellow and add tiny bits of the other colour till you get your desired result

 

When mixing with red, do so sparingly, as it’s so strong.