If it's butter cream icing it always turns out pink & the more food colouring you add the runnier it gets. We went to a party that had a spider man cake and it stained all the kids hands & faces and was soooo runny.

the idea above is a good one, or I heard somwhere (look it up becuase I could be way off with this info) but the tip was to use copha instead of butter as it's white not yellow like butter. Good for white icing, red or dark blue but has to be left out to go nice and soft.