first i have to ask what the fiber content of the yarn is.. if it's mostly wool.. you're good..
cotton won't dye with food dyes and i wouldn't try on synthetics
soak the yarn for at least 10-15 minutes before you add the colors.. that way the dye will have an easier time of entering the yarn more evenly
don't forget your acid.. vinegar! the color won't stay without it
and be sure to have fun!