Most people are taught at school that there are three primary colours; red, blue and yellow. And that mixing any two of these, creates the secondary colours of green, orange and purple.
Grab your brush and dip in in the yellow, wash out, then the blue: magically, green appears. Hey, this is easy!
The yellow and the red this time, orange is easy too!
But how many of you have mixed red and blue and got a manky reddish brown colour instead of the purple you wanted?
Where did you go wrong, you wonder, and hide your work from the teacher, believing this was proof you had not an artistic bone in your body.
Or, having more confidence, you ask the teacher to try, and the teacher, rather embarrassed, gets the same (only usually neater):
So, why the brownish plum? How do you get purple?
Unfortunately the teacher was at fault for not being able to explain (or possibly even knowing why the red (or blue, or both) supplied was incapable of making a purple.
Here's the reason:
To make a good rich royal purple, you need a red that is more crimson than scarlet - ie one that doesn't have too much yellow in it. (Save the scarlet for making your shades of orange)
Now mix this crimson with a reddish-blue, that is, one that is not too green... ultramarine is good, cobalt no good...
Now, you'll be able to make a number of good plums and violets and purples.
Greens and oranges are not quite so crucial. The crimson red does make a reasonable orange, and the reddish blue makes a dullish green.
To make it easier, let's just say there are not three primary colours, but six: two primary reds, two primary blues and two primary yellows.
I'm convinced that children in art classes at school should be provided with two of each, and allowed to experiment with mixing them in a systematic way so they understand that you cannot make all three secondaries from only three primaries.
The fun should be in the exploring!