The basics are that you have three primary colours - Red, Yellow and Blue, then you have black and white. With these you can combine them or as 'nungate' says 'play' to see the results. As you are planning to use water colours as opposed to oils you can also use water to dilute the colour and make it 'weaker' by adding water or more intense with less. Most colours are obtained from mixing any two of the three primary colours, combining all three will create a brown. Good luck