There are no markings or hallmarks that are specific to white gold as opposed to yellow or rose gold. Jewelry markings or hallmarks typically include the metal stamp or carat weight, the manufacturer, designer or any trademark.
Carat weight refers to the purity of the gold, not its color. Pure gold is 24K or 24 carats, and other carat weights contain gold along with alloys. The alloys are what create the various colors of gold. For example, 14K yellow gold and 14K white gold both contain 58.5 percent gold. Metals typically used in a white gold alloy include silver or palladium.