the nite shades might be more trouble then it is worth
the spray usually gives a patina look to it. usually you have to do the niteshades spray, wet sand then hit with a clear coat. a lot of work for a small area. (this is of course froma year old experience, they may have changes formulations I.E. roberts case)
I think (brainstorm here) it would be easier to get a sheet of auto tint from pep boys or somewhere and tint the exterior of the turn signal that way. you ill also get a blue black as the nite shades is a brown black
i know of a company that will do the taillights or anything you send them relatively cheap, they sand and prepe the peice first so the spray (which is a paint) doesnt flake or peel.