You should be able to find epoxy pigment at most plastics stores and I'm sure boat/marine suppliers will have some pigments too. They usually come in liquid or paste forms for about $5 for a small bottle. I would recommend the paste form if you're looking to create a colored epoxy that is opaque and the liquid form if you want a transparent look.
I use refill colours for copic or pantone(tria) markers. you have an endless variation of colours and they are very easy to mix. Great for transparent. Never experienced any bad influence on the bonding or strength.