I think if you paint the membrane an warm brownish orangy colour and put over that a semi transparant layer or layers or drybrush of neutral grey(a thiny bit of blue+red+yelow+a lot of white and black)you'll end up with an optical neutral purplish/bluish grey that looks spot on.
Make sure the underlayer of brownish orangy colour is darker than the grey you put on top or of the same darkness as the grey,..not lighter or it will not produce the in this case desired neutral optical grey.
Aproach it opticaly mixture wise /colourwise/colourtemprature wise like this;warm over warm=warm,..cool over cool=cool ,..warm over cool=warm ,..cool over warm=neutral/coolish.
Light over dark=more neutral/cooler ,.dark over light=more neutral/warmer(except if you put a darker cold colour over a lighter one,..but the effect is "warmer" than if you painted the darker cold colour without the lighter underlayer.
"warm" are the colours that lean towards yellow/yelloworange,."cold" are the colours that lean towards blue/bluegreen.
In the folds then you could paint a bit of transparant purple and a bit on top of the folds to make the whole membrane look even more transparant,...don't highlight the raised parts or shadow the folds,just make the most raised parts a bit more neutral/colder and the deepest parts a bit more warm to enhance the highlight and deepest shadow..
The last step(enhancing highlights and deepest shadow could be skipped though since the sculpted folds will take care of the deepest shadows and highlights it self
All this optical grey will also make your colours of the t-rex look more vivid.
It is nice if you put tiny spots or spatters of this optical grey on your whole model to unify the whole colour scheme.