I used to have that problem when I started decorating cookies. What I was doing wrong was that I would mix and beat the merengue powder, water, cream of tartar and sugar all at the same time. Now, I beat the powder with the water first let it get foamy and then I add the cream of tartar and sugar. By doing this my colors don't bleed anymore. Hope this helps you out too.
10/6/07 at 10:07pm