You can buy colorant at places like Delphi glass. Some have reccomended tints-all which I think art supply places sell and some have used acrylic paint, but I think you need to add ALOT to get the color you may want. Home depot and Lowes carry some colors of grout and you can usually get a smaller bag. Try tile stores, too they carry smaller containers

I'm not sure of the shelf life of grout. It's probably ok if it is in a cooler dry place

I only ever buy charcoal and white and use the colorants that shrty mentioned. I will mix them with charcoal when I want dark tones and white when I want light bright colores or pastels depending on how much colorant I use. I can get them as seperate colors here and she gives this forum a discount if you type gardenweb in as the discount code

Here is a link that might be useful: maryland mosaics