hey, wait a minute.... there's a way to do that IN the sketch renderer... hmmm... okay, maybe not exactly. down at the bottom of the sketch designer is a weird slider named color blend. and what it does is stick a copy of the current display mode on top of the sketch render, with opacity from 0 (none) to 1 (100%). but it doesn't have the same detail and control you get from doing two renders and splicing them together. never mind. :)