: : : Most illustrations show him as having brown skin but as a Hobbit (albeit one 'twisted' by bearing the One Ring for 900 or so years) I would presume that he had white skin to start with. He avoids the light so he wouldn't have a tan.

: : : Can anybody add any more information?

: : Hmmm, if he would have been white, he would have been far more easily spotted in the dark.
: : Maybe, but this is really a daring hypothesis, he had chameleon-like abilities?
: : Doc

: His natural pigmentation would probably have been pale because he avoided the sun and even the moon whenever possible. If he was described as being dark colored I think it was because he usually was covered in filth and grime.

As I remember it the Fallohides were the only conspicuously pale skinned of the Hobbit groups. The ones Gollum was a member of, I think, had a nut-brown colouring. I've always thought of them as resembling farm-workers who develop a weatherbeaten year-long tan.

It is a guess as to how centuries of living underground might affect Gollum's original skin colour. Although he avoided direct sunlight, it is possible that he may have returned towards his original colouring after his emergence from the caves. This could, possibly, explain some of the different descriptions.
What effect the Ring might have had on his colouring is another unknown, as also is the possible effect of the sudden removal of the Ring's direct influence. Given his age - his hair might or might not have been grey.