I'm not big fan of the silly putty technique for hard edge camo. If you not careful, a ridge of paint can develop along the silly putty edge. When I do hard edge camo, I use Tamiya masking tape. The trick is to start with really thin strips to outline the camo. The thin strips allow a lot of flexibility in the tape. I cuts strips about a 1/16th to an 1/8th thick. After outlining the camo, I go back and fill in the area not to be painted with larger strips. After painting, some touching-up will have to be done especially in tight areas. That's easy to do with a brush.
Here's a few shots of camo done with masking. Notice all of the tight areas along the ribbing on the sides:
-Jesse