Indeed there are formulas, or guidelines, for mixing colors, and if you do not do it properly, you will end up with what is called "mud."
First of all, as far as the color wheel goes, it's purpose is to show us that colors across from each other on the color wheel "complement" one another, give us colors that not only enhance each other, but have a great deal of contrast. Red and green are across from one another, yes, but if you mix complements you will get a shade of gray. In other words, complements will neutralize into gray. You will NOT get maroon. If you wish to learn more about the color wheel, a google search will bring up a considerable amount of information.
Secondly, you will want to use primary colors to mix, or again, you will chance to make "mud."
Now, do you want a warm shade of maroon, or do you want it to be cool—something to consider.
Cool: Start with a primary red color, of course. Add a small amount of primary blue (you can always add more later, but too much will make purple), and then add black until you achieve the tone or value of maroon that you desire.
Warm: Same as above, except instead of the blue you would use a small amount of primary yellow to achieve a warm maroon that will have a hint of a brown color as its base (you can not add brown itself to it, as it is not a primary color and will create mud).