A perennial question to demonstrators at our art group is, "What colour do you use for the sky / grass / trees / water / etc".
Most experienced painters will have a favourite palette of colours which they will use all the time. from the dozens of colours available however it seems that the same few pop up again and again. Often it is better to use a limited palette to achieve a uniform overall appearance rather than selecting one colour for each item painted. It is possible to mix any colour, within reason, from the three promaries which your palette contains. I have written another Squidoo lens on this very topic. Which three primary colours best exemplify the seasons for those of us who prefer to paint landscapes.
The sketches above are used as an example of the main combinations, why not find time to take a quick look?