I've noticed that ALL acrylic sealers make the colores of the pastel more vibrant. Some store- brand sealers don't cause that.
You can try mixing Future and Lighter fluid for a sealer and using an Aribrush. It creates a nice finish and dosen't alter the colours at all.
If you want to experiment, try some hairspray.Hold it back about 2,5 or 3 feet though....or else it will look like the sculpt just got dunked in oil.
'Only two things are infinite -- the universe and human stupidity, and I'm not so sure about the Universe.' --Albert Einstein