Well, maybe not 'common' in the sense that every other Elf would have it, but as in 'not uncommon, not exceptionally rare', which amounts to about the same thing as your 'found among them occasionally'
. But who knows - perhaps even the dark-haired among the Teleri had a sort of silver-metallic lustre in their hair, counterpart to the dark copper-red found among the sons of Fëanor?
Which raises the question: Could it be that the name
Sindar referred in part at least to the (cum grano salis
) 'commonness' of that hair colour among them, especially in the royal family (beside the obvious sense that they were Elves of the Twilight in contrast to both Calaquendi and Moriquendi)? After all, the Vanyar were also named thus for their hair colour, weren't they? All speculation, I know...