[GregS]

Yes, I have used it, and as far as I can tell, it is clear. Certainly, when I apply it to a white applicator pad, I do not see any colour at all.
FWIW, it's early days yet, but it is definitely still on the paint after 6 weeks, with no sign of any degradation.  My initial impression is positive. It has survived two touchless automated washes, which use reasonably strong chemicals.  (a subject for another thread perhaps!)

[Corribus]

Just a quick note to add that nanoparticles of certain sizes do not efficiently scatter visible light, so a nanoparticulate suspension would not necessarily be opaque.