I just did a camo cake on Tuesday. I didn't pipe on mine. Well, I did, but then I started over because I just couldn't see how I would get all of the lines out. DH wanted me to outline a pattern, but I didn't, and I think it was easier that way.
I just used an angled spautla, dropped a blob toward the center, spread it out a little (without making it perfectly smooth), wiped off the spatula, picked up color #2, dropped it next to the first blob, spread it out, repeat with color #3. Soon, I was working with one color for a while before having to switch to another color. I used toothpicks to help the icing get into tight spots. It was not smooth once it was all covered, but the buttercream was mostly even. I then used the Viva papertowels to smooth it.
Oh, and I used Wilton Gel Colors, too. The only Americolor I used was black (yellow and black, med. green and black, med & dark green and black.