Hi

I have numerous coloured cells on an excel worksheet and I want to make a formula to count the different colours... ie all the red ones, the green ones etc...

Which formula would I use

You'll need to use Excel 2007 if you plan attempting this with any builtin function. If you're on an earlier version, you'll have to add this capability into your worksheet by installing some UserDefined functions.

Chip Pearson has written many many functions like this, here's some:
Conditional Formatting Colors