I never heard of using cake frosting itself to dye (isn't there way too much sugar in there?), but I know some people have used the gel dyes that are used to tint frosting - Wilton's is the brand that comes to mind. Any craft or other store that sells cake making supplies would have it - JoAnn's, Michael's, etc. It's very concentrated, probably has little to no sugar in it and I think is only good for natural fibers (mohair, wool, etc., but not cotton).
The color in frosting would be way to subtle. Food coloring, and the paste coloring made by Wilton + vinegar works great though. I've also used Jell-o, but again the coloring was way to subtle for me. The sugar rinses out, btw, so it's no biggie.
Thanx everyone...now that ya mention it it wasn't cake frosting Does anyone have any recommendations on dyeing roving and yarn with these. I tried food coloring and vinegar on some roving and most of it washed out, any ideas as to why?