Depending on your metals carat (22,18,9ct etc.) some metals can be alloyed down or mixed, but it is on a case by case basis please get in contact if you have any questions.
Some people will Rhodium plate yellow gold to give it a white gold look as in most cases white gold is Rhodium plated to give it its beautiful white colour, but this is a short-term solution and when it starts to wear it is very obvious as the colour contrast is much more obvious than white gold.