Regular Food coloring is generally not recommended as it may transfer and stain your skin when using the soap. The color bases sold online are specifically made for soap-making, so the dyes will not transfer to your skin and the fragrances usually last longer. 🙂
Check out Hobby Lobby or Michaels, I know I have seen coloring for soap at Hobby lobby when I was looking at candle making supplies.
Check Amazon or retail stores like Wal mart or Michael's. They carry several different colors and scents.
Yes you can but too much of it can leave stains on your body and bath, so just keep that in mind :)