I did this one recently, Cake is covered cream cheese frosting (with marscapone and whipped cream). I thinned royal icing down with a little water and used a new toothbrush. The icing consistency has to be drizzling off the brush when you dip it. Run your thumb hard down the bristles to get finer spots. Stand a little back and flick your wrist to get a good splash and lines. When I tried to flick it on with a paintbrush it didn't work for me. The toothbrush worked very well.
The only thing is it did "bleed" just a very very little into the frosting. But didn't run or affect the look, and you could only see if if you really looked hard.
p.s. Be very careful on choosing your colours. Some colours mix to make muddy brown etc,
Hope that helps!