Food Coloring in Soap
Can you use food coloring in soap?
Some of the powdered dyes I have bought were food safe colours and they did work in cold process soap making. Just remember that dyes will bleed so if you try to marble a soap there won't be a defined line between the two colours.
I'm not sure how well the food colouring that we can buy in the grocery store will work. It may not be strong enough and some of the colours (like blue) might not be able to survive the saponification process.
Visit my page on Additives
for a great selection of the different types of things you can add to soap that will lend texture and colour.