You can make every color under the rainbow (or in it) from just three colors of paint.
Don't believe it? Here's how it works. There are three main colors. Red, Blue and Yellow. Every other color is a combination of two or more of those.
The other two crucial pigments are white and black. With those five base colors, any color at all is within your grasp.
I use acrylic craft paint a lot, but this works with every kind of paint.
In the hardware store when you buy paint, you'll see this at work; you choose the color you want with a 'paint chip'.
This has numbers on it usually, or the technician uses a computer to get the 'recipe' which tells them what base to use, and how many pumps of each colorant to get the hue you desire.
After a good shaking, the paint is ready for you to use.
I don't mix my paints completely, preferring the more rustic look. In fact, I usually just squirt several colors into a small plastic container that I've saved for exactly this use (yogurt, cream cheese, etc).
I don't mix them, just dab my brush or whatever other tool I'm using (could be a rag, or a piece of sponge).
There are lots of ways to make your items look aged, from using giggle juice as the liquid used to dilute them, to adding a touch of green paint.
Yes, I've done that on occasion. Don't have green paint? Use a dab of yellow and a dab of blue.
Make red barn paint from giggle juice and Holiday Red paint (or any other red pigment).
My Mum was a set designer at a certain time in her life, shortly after completing her training as an art teacher.
She's told me about developing 'donkey brown' which is all the different colors of paint you can find, dumped into a bucket and stirred. This can be used for many parts of a stage set, from timbers to stones.
The caveat here of course is that the paint has to all be either oil based or water based or it won't mix together.