How can I make my hair color last longer?

I need to color my hair every 4-6 weeks because of gray hair. I like getting it professionally done because it comes out better. But it comes out more expensive because of how often I have to color my hair. I use shampoo and conditioner that are for color-treated hair and my stylist advised I wash and rinse in cold water (which I try to do, but now that it's cold in the mornings, that's hard). Does anyone have any suggestions or recommendations. Thank you!How can I make my hair color last longer?
I use this Treseme' color lock stuff for my color hair and it really does work a lot! Also wash your hair in luke warm water because hot makes your hair fade a lot