: : :As his father Eol, a kinsman of Thingol, was called the "Dark
: : : Elf" perhaps there were Telerin elves of somewhat darker
: : : colouring than others, though all were dark haired.

: : Thingol's hair was silver or silver-grey, and several of his relatives were said to have similar hair (Celeborn and Cirdan among them).

:
: I had forgotten that, though these persons may have originally had dark hair if analogies with humans can be drawn here.

:
: : The unusual color for Maeglin's eyes may not have been all that unusual. It wasn't until the publication of THE PEOPLES OF MIDDLE-EARTH that we learned there were auburn-haired Noldor (some of Feanor's sons among them).

: Yes, Tolkien does seem to have relaxed his originally fairly clear-cut physical/racial traits over time. This may be connected to changes in the understanding of how heredity works which took place over his life time. The origins of his use of the grey eye with dark hair combination for the majority of elves he describes is fairly obviously drawn from the same characteristics in his wife - who seems to have been a considerable source of inspiration in Tolkien's appreciation of beauty.

: It is also notable that Denethor is the only Dunadan described as having dark eyes. The note about many Beorians having brown eyes in The Peoples of Middle Earth seems to be a case of Tolkien retrospectively validating his earlier description - possibly unintentionally.

Denethor also was of mixed (Adunaic and Middle-Earth) ancestry, not a pure Dunadan, though it was said that in him the Adunaic traits were very clearly manifested.