No wonder people get to believe colour theory is hard. I came across an article describing secondary and tertiary colours as colours mixed “in equal concentrations“. How would you measure it? Is it equal in volume or tinting strength? What fearful disaster is going to happen if the mix is not perfectly equal?
Keep it simple and straightforward. There are three primaries: yellow, blue, and red*. You mix two primaries together to get a secondary: blue+yellow=green; red+yellow=orange, red+blue=purple. When you’ve enough mixed blue into a yellow that it looks more green than yellow, then you’ve successfully mixed a secondary. The shades of this secondary will vary on the proportions of the two primaries. It will also vary depending on what particular blue and yellow you mix.
The word “tertiary” comes from Latin tertiarius/tertius meaning “third”. Mix three (or more) colours together and you’ve a tertiary colour. Those browns and greys that are ‘interesting’ colours when you do it deliberately and ‘mud’ colours when you do it inadvertently. (Remembering that a secondary colour is two, and what’s in a tube colour may be a mixture of pigments.)
Quit fussing with blue-green and yellow-green or red-orange and yellow-orange, giving names to variations of primaries and secondaries. Forget the colour theory that insists you quantify these variations. Keep the theory simple. The colour-mixing journey is a lifetime’s; we’ve so many pigments available to us, giving so many possible combinations. Working with fewer colours and internalizing how these mix, and practice, is how you mix the same colour again and again.
(*For CMYK followers, cyan counts as blue, and magenta as red.)