I was asked recently at what age children should know their colours. Like most things it depends how much instruction goes in, and how frequently a child has the opportunity to play and practise.
Around the age of 2, children are learning about colours. You can aid this by mentioning colours as part of your everyday conversation eg:
You have your blue jumper on
There are yellow buttons on your shirt.
You’ve made an orange painting
You are eating the red apple and so on.