Depending on the proportion of the colours used you get different shades. So f.ex.:
- If you want to make green and you add more blue than yellow you'll end up with a blue-ish green.
- If you add more yellow than blue you'll get a yellow-ish green.
I think this is the most crucial ability when it comes to mixing colours - the ability to see how much of a certain colour is in a colour.
Look at the purples in the photo... Can you see which one has more red in than blue? and which one has more blue than red?
Take a look at the oranges... can you see which one has more red in? and which one has more yellow? If yes, great! If not, don't worry, you'll get there! it's all just practice! :)