White gold does not tarnish. However, rhodium-plated white gold discolors over time as the finish wears away. Rhodium gives white gold items a bright white appearance, but when the rhodium plating erodes, the yellowish cast of the white gold underneath becomes visible.
Tarnish, as a chemistry term, is a chemical reaction between a metal and a nonmetal that results in a dull, darkened area on the surface of the metal. It is especially associated with oxidation, a reaction between an element and oxygen. Because the yellowish tinge caused by rhodium eroding from a piece of plated gold is due to physical wear, it is not considered tarnish in this sense. The white gold underneath the plating is not altered. However, pieces of white gold that have eroded plating are said to be tarnished in the sense that they have become discolored. This is a colloquial use of the word tarnish.