Goes without saying , the fellow in the email (and a few others out there, especially that damned judge in the figure painting competition) seem to be;
a) ...projecting a degree of their own beliefs/political leanings/fantasies into the setting on a subconcious level, and making domain assumptions that coincide with this foundless set of assumptions.
b)...are total douches.
Reading this brought to mind an old Pratchett quote.
"Racism was not a problem on the Discworld, because -- what with trolls and dwarfs and so on -- speciesism was more interesting. Black and white lived in perfect harmony and ganged up on green."
Kinda appropriate for the 40k universe.
Also with so many things to be scared of already, who has time to start making up new ones?
As to how far skintone could go into the more exotic before being considered mutation?
All depends on the stability of the trait I'd imagine.
If such a trait was present in a population, due to environmental alteration and/or amplified as a result of isolation prior to explorators reconciling them with Imperial control, then I'd be inclined to think that after a certain degree of testing by the magos biologis, designed to appease the more hardlined members of the ecclesiarchy present (or paranoid interested inquisitorial parties for that matter).
Combined with close scrutiny of the populous belief structures and how well they adapt to or conform with ministorum teachings, judgment on whether the alteration would be considered part of His plan would be made.
Though on saying that, be far enough away form home and a blue skinned guardsman could get the type of treatment reserved for mutants and the like.
Just my take on it of course.
Makes me wonder though, we know that there is a certain degree of mutant underclass in 40k, and some worlds even rely on it to an extent, but what level of traits and alteration of form would have a chance of being 'sanctified'?
Would it stretch as far as minor alterations of body morphology like the ratlings, ogryn and the late lamented squats? or would we be looking at even the smallest changes, however stable, being viewed with suspicion and fear?
(also, a shiney new throne for the forum techsavant who tells me how to singlespace this Emperor forsaken post.)