I think of it as a minimal form of synethesia-  the scrambling of sensory input in the brain-  so that sound has colour, or colour has taste.  For me,  colours are a combination of scent and flavor;  some make perfect sense  (like mint green being cool and dusty) and some are capricious (like purple being-  not grape-  but a sweet yet tangy earth scent).   It's not a process I think about, really,  just the only way I can explain it in ways someone who maybe has never considered it can, hopefully,  understand.

Now according to colour theory,  yellow and blue do not truly make green-  and that's just fine with me because I rarely want to make green;  usually,  I want to cool, or warm, or gray, or just flavor an area with a green rather than "make green".   In the past two years, my usual palette has shifted from mostly pure colours to mostly greyed colours-  I've discovered that a subtle bit of greyed colour sings a clearer, less clashing note-  the passage of colour is more harmonious-  than a high-chroma'd pure colour.  This means stroking in an area of warm pinkish gray gives me a more harmonious, less garish area than if I'd clumsily laid in a swatch of pure pink.

Am I making any sense at all, yet?  Probably not.  So....

Let's just start with the two best-known colourwheels.

Yup,  two.  The first is the traditional one we all learned in school:

With this as a guide, you can easily see what are called "the primary colours"  (red, blue and yellow) and also "the secondary colours" (orange, green and violet).  You can also see the "complements"  (note spelling)  of each of the colours given:  Red's complement is green,  orange's complement is blue, and yellow's complement is purple.   Complements in this discussion mean two things:  First,  a complement is the colour directly opposite on the colourwheel, and as such, the two seem to exaggerate each other,  and second, they can be used to grey each other- or effect the chroma of each other from pure to less pure.  (Chroma is a relative measurement of how 'pure' a colour is-  relative because what appears 'pure' red-  not orange, or pink, or maroon- to me,  may appear rather orange, or pink or maroon to you.  It may also appear dulled-  as if there was a haze or fog of smoke over it, dulling the colour-  greying it.  Grey in this case doesn't refer to the colour gray,  but to the relative chroma of the colour present).

Are you lost yet?   It's tricky stuff to understand in words, I know.  Trust me, though,  once you practice a bit,  it'll become second nature-  and then YOU can try and figure out the words to explain it to someone else!

The colourwheel above is a perfectly good place to start learning about colour and how it works-  but there is so much more.

So, let's move onto this one:

This is called The Munsell Colourwheel,  and it is considered the more sophisticated and exact example of colours and their complements.   If you think about it, it's rare you paint with pure or high chroma'd colours-  usually, if you use, for instance, a green,  it is an olive, or a blue-green, or a turquoise, right?  Well,  if you think about it further, an olive green-  which is 'made of' a green and an orange- would not have the same complement as a blue-green, would it?  Couldn't have-  while green's complement may be red,  orange's complement is blue-  which would make olive's complement a violet, (red and blue) wouldn't it?  And in the case of blue-green, green's complement is red, and blue's compliment is orange,  so blue-green's complement must be a red-orange.  Thinking about it like that it makes perfect sense.  It makes even more sense when you lay a pencil or brush handle over top of one of those colours up there and see what lies exactly opposite.   You'll also note the Munsell wheel doesn't differentiate between primaries and secondaries-  that's because, bottom line,  there's no such thing.  I know-  it goes against decades of teaching, but it's true.   Primary and secondary colour theory is just a convenient, easy way to explain one of the behaviours of physical colour-   but it isn't really true.