Ah! I can answer that one. Since one of the parents was Drow, that one is easy. (Harder to give a definite answer if it was a coupling of two different surface elven subraces).
From Drow of the Underdark (2nd ed)
[quote:11psqieb]"Half-drow" usually breed true back into the drow race.[/quote:11psqieb]
From Demi-human Deities (2nd ed) in Vhaeraun's writeup:
[quote:11psqieb]Contact and marriage with other elven races is encouraged. Half-drow usually breed true back into the drow race; Vhaeraun sees this practice inexorably raising drow numbers in surface lands.[/quote:11psqieb]
There is a slightly pared down/editted version of the above in one of the 3rd ed sourcebooks too. I can't remember if that part was snipped out of Vhaeraun's whopping 2 paragraph write up in Faiths and Pantheons or not. But Lords of Darkness talks about it too.
So in a pairing of a surface elf and a drow, the child will most often take after the drow parent, but if you want to be one of the not-so-common exceptions, you could take after the surface elven parent, or be more like a drow/human half-elf, a blend of both.
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For a moonelf/drow pairing specifically, my personal preference if I was your DM would be for the child to have dark or light greyish skin tone, a bit darker than the normal moon elven tone, and towards the lighter side of the normal spectrum of drow skin tone (which can range from dark greyish to obsidian/jet black.)
According to the old Drow of the Underdark book in regards to the rare drow who have blue or violet eyes, 'this usually denotes some surface elven ancestry somewhere in their family line'.
So a half-drow moon elf should definitely have one of the eye colors that are common to moon elves but non-existant or exceedingly rare with drow. Same with hair color, make it silver or white if you wish, or give one of the colors common to moonelves... or even a combination of both.
I'm a firm believer of PC's being the rarer exceptions than the norm, but you COULD also just say the child looks fully or mostly drow, if you wish too, since that is what 'usually' happens with these crossbreedings. But a moon-elf/drow pairing probably offers the most unique and cool appearance options, imho.
I played for many years a half-drow half-gold elf. He looks drow, mostly. Surfacers usually mistake him for a full drow. But to drow, they can always tell he is a dirty half-breed with 'tainted fairy elf blood' - his skin tone isn't quite dark enough, and more telling is that his eye color came from his gold elven mother, and his white hair is marred by a single thick jet black lock of hair on the left side. They know. Some gold elves and moon elves he's met also were able to visually pick up on the fact that only one of his parents was drow, the rest either don't notice, or refuse to admit it (usually the case with the haughty gold elves).
Hope that answers your question.