What is the ruling on wearing white gold in a suit button, knowing that it is half a gram?

There is no harm in wearing the suit mentioned in the question, because a small amount of gold that is subservient to another material is permissible according to the preponderant opinion, which is the view of the majority of scholars. This applies when it is pure gold; how much more so if it is mixed with other materials, or if it is entirely platinum. White gold is a mixture of gold with other metals such as palladium, nickel, silver, copper, iron, or aluminum, which affect its color.