I have dark brown hair and recently i noticed that some of the hairs
are yellow-goldish color. Why is this? They seem have a buildup of
something because the yellow stuff can be removed but with much
difficulty. It is affecting some new hairs and some long ones. I am a
very clean individual, and relatively healthy. Please help! Thanks in
advance.

Dye from new underwear?

Sounds like a fungal infection.

You could try an antifungal shampoo in the affected areas for 30 days.

See your doctor of course.

trichomycosis axillaris is your aanswer. take a minute to look it up
on emedicine.com google or wikipedia.  it is no big problem. nothing
to worry about. pass it on