Does it look good if you wear white gold necklace with a gold dress?

I don't think that there is anything wrong with mixing white and yellow metals or, in that case, white metal with yellow materials and vice-versa. The thing to bear in mind, perhaps more than colour, is the texture and weight of the fabric compared with the size of the jewellery. For example, I advise many Mothers of the Bride/Groom with their wedding jewellery and would steer someone towards a bold choice of collar if the dress material were something substantial such as a brocade. The style of your dress, ie. neckline, and your physical shape - would a large necklace dwarf you? - should also be considerations.