There are three primary colors when working with pigment- which is what most color theory is based on. You can thank painters for all of these ideas.

So the three primary colors are red, yellow, and blue
From these three all hues (pure color) can be made
From pairs of the three, the secondary colors can be made

Red+yellow=Orange
Red+ Blue= violet (or purple, though thats not the artsy term and purple is technically a different color)
Blue+ Yellow=green
This makes up the most basic color wheel:

And if you mix the adjacent colors again, you get the tertiary colors. There are six of these and they are named less creatively with the primary color involved followed by the second color involved

Yellow-orange
Yellow-green
Blue-green
Blue-violet
Red-violet
Red-orange
And this gives you a more standard color wheel

Though they can still be mixed further to form colors like red-red-orange and other crazy shenanigans.
The next few posts are on classifications of how the colors interact strikingly, how to use it, and how to make colors that aren't pure colors, but this is where everything starts-mostly.