I'm sure it's been already written somewhere,but I can't find it,so... :P
To get a pure color on a foal you have to breed two ponies with exactly the same colors of their manes, coats and eyes. It can be only one identical color, but all three can be identical. For example, if you breed a mare with Yellow slightly mane, Denim medium coat and Red slightly bright eyes with a stallion with Yellow slightly mane, Denim medium coat and Red medium eyes, you have a chance to get Yellow slightly PURE mane and Denim medium PURE coat on the foal. But double pure colors appear rather rarely. It's not so easy to get a single pure color too, so what you need in the first place is patience ;D
I hope this helps. Good luck with breeding pures! :)