You probably learned this is elementary school but in case you forgot.
Red, Yellow and Blue are your primary colors
Blue+Red= Violet (or purple)
Orange, Green, and Violet are the secondary colors.
The colors mixed in between those are called tertiary colors.
(Note: Just in case you feel like being smart with me I'll point out that we are mixing paint, not light thank you very much. Find out more about mixing light here.)
2. You need to know that color has a temperature
When I think of color temperature I think of orange as the warmest and blue as the coolest and everything else is on a scale in between. Thus red can be warm or cool depending on how much orange or blue there is in it. Same with the other colors. There are other ways to explain this but my brain likes this way the best. If you have something to add feel free to comment below I'd love to hear other perspectives about temperature.
3. Color has opposites
Look at the color wheel and pick a color. The color strait across the wheel from it would be it's opposite or complementary color. Red and green are complementary colors so are blue and yellow and yellow-orange and blue-violet. Get it?
Also if you mix a color with it's opposite you neutralize Them. This means you get a sort of grey or brown. If your getting muddy colors in your paint and you don't want them, make sure you are not mixing complementary colors. Likewise if you want a more subdued red see what adding green will do.
4. You need to know what a "shade" of color really is.
We often say "a shade of red" if we mean it is a red color but not red red, but that is incorrect use of the word.
Shade actually means a color that is mixed with black. So a shade of red would actually be a darker version of red like maroon.
A Tint is a color that is mixed with white. So pink could be considered a tint of red
A tone is a color mixed with gray. I don't have any color examples for this. Any ideas?
For more about color theory there is some pretty good stuff on Wikipedia. If you have any questions about what I've said or have anything to add that has helped you in painting with color leave me a comment.