What colour you render over has nothing to say. All that matters is that it's saved in a format that supports an alpha channel. And yes, png does that too. Personally I prefer tiff as I can use that in all sorts of programs, and it's totally lossless.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.