1. Use sulfate-free shampoo
Pretty much every "normal" shampoo contains sulfates (sodium lauryl sulfate) and this chemical is fine for natural hair but it really fades the pigment in coloured hair. So a sulfate-free shampoo protects colour and doesnt contain the chemicals which fade it.
2. Use a conditioner specifically made for your hair colour
A conditioner made for your hair colour will help to maintain the brightness of your colour.
3. Wash your hair in cold water
Not only does cold water make your hair more glossy but it helps to maintain your hair colour too. If you can't bare cold water it is fine as long as it isnt hot, because hot water can break the hair shafts and fade the pigment.
4. Rinse your hair in herbal tea
This one sounds a bit strange but before you wash your hair, rinse it through with a herbal tea that matches the colour of your hair. So for a bright red hair colour use hibiscus tea as it is a vibrant deep pink tea, for blonde hair try chamomile tea and for copper hair use a ginger tea. They help to keep the colour bright.
I've never been too good with looking after my hair colour and it always fades, so I'm going to give all of this a go and hope it works!