I was pleasantly surprised to discover that all it takes to create vibrant colors in Oogoo is very small amounts of linseed based oil paints. Apparently the pigments in oil paints are very fine and dense so a little goes a long way.
About the equivalent of 5 or 6 drops of oil paint per teaspoon of silicone will result in solid colors. I mix the color well with the silicone before adding the corn starch.
While you can leave it white, it is a good idea to add some color to make it easy to see if you have a well mixed batch. Adding color does not appear to significantly effect the hardness or flexibility of the final result.