How to add colour to grout for wall tiles
Just a guess, i would look into dyes or pigments for concrete . 

Mix it into a white grout. experiment with colors and ratios, take notes so you can scale up when you find the combo you like.

A quick web search found this web site as just an example. 

I Also found this tutorial, 

But wait, there is more. If you act now i will include a link to a handy search engine where You also can use the internet to find out things.