You don't really need to decide your grout color until you have your pieces (tesserae) on. Grout can really change the appearance of your piece, so it's always fun to see what happens. I sometimes lay out some of my tess on pieces of plastic in the grout colors I'm considering and then take photos of my options. Good luck and send photos for sure!

As DAISY said, your main concern now is not grout - it's adhesive and substrate. What kind of table? Are you planning to use a concrete-based adhesive (thinset)? Give us a picture, please, so we can help. If you are cutting your tiles, Hobby Lobby carries stained glass, and every now and then they put it on sale for 25/30% off, which is the only time I buy it. Maryland Mosaics, Wits End, Mosaics by Maria are three companies lots of us use when ordering. You can Google on-line for tons of tutorials, glass companies and any other info on mosaicing, if you're a beginner. Of course we'll be happy to help along the way. A book is a good start too - from your library.