The basics of colour theory
"Ugh, but mister, this is soo boooooooring, why do I have to read theoSILENCE!"

Theory is important for pushing yourself further, for without knowledge, how would you know what to paint?

You have already amassed some knowledge, and (hopefully) you've seen a lot of colourful things in your days, now it's time to understand some more baseline stuff, and then move on to converting that knowledge into actual paintjobs.

First, colours have certain properties, the most important ones (for right now anyway) are temperature, saturation and value. They also have relations to each other, we'll only touch on complimentary colours in this piece though.

A simple colourwheel

As can be seen on the above colour wheel colours belong to either the cool or the warm side, this is of importance for multiple reasons, amongst others contrast and atmosphere. This is what's known as temperature.

Colours opposite each other on the colour wheel (indicated with some very helpful arrows) are called complimentary colours. Complimentary colours can be used for contrast, and they're excellent to use for shading and desaturating colours.

"What's desaturation?" 

Well, I'm glad you asked (this is not a forced segway, shaddap). Saturation refers to how colourful a colour is, or, how grey it is.

A saturation gradient

The above picture shows the difference between a very desaturated red (on the left) and a saturated red (on the right).

Using too many saturated colours on your piece can give it a very clown-y feel, whereas only using desaturated colours can give it a dull, grim, boring feel. The best is a balance, experiment and find out what you like. Maybe you really like lots and lots of bold colours, then more power to you! This is not exact science, it's very subjective, so do what you like. :)

"Alright, so what's value then?"

Well... value is how light or dark something is, usually represented by a black/white gradient. White is the brightest colour you can paint, and black is the darkness, this stands true for every colour. 

The easiest way to see the value of your paint is to take a black/white photo of it (most smartphones can do this) and look at them. A lighter paint has a higher value, and a darker paint has a lower value.

Value is very useful for shading and highlighting, or describing form by suggesting light and shadows with different values.

A rendered sphere

Here you can see an object that looks three-dimensional, despite your screen only being two-dimensional. This is due to the use of values to describe shape. Every shape consists of surfaces that have their own behaviours when exposed to light, so look at objects around you and see how they behave!