A gold Base under Kandy red gives a darker tone /tint , a sliver base gives a brighter tinted red and bit more vibrant ( and depends on how many coats are applied too ) , I use a high metallc charcoal under my kandies as I like the look better , a bit darker then silver but not as muddy looking as a gold base IMHO .
YOu can use a pearl white base , or a pearl black base , heck I have shot it over a Bright Metallic red before , gives it a nice look also .
There really is no standard color , but Gold & Silver are most commonly used .

Go to a local paint jobber that sells kandies and see if they have a kandy color chart to look at , as they usually have several kandy colors shot over different bases , look at it in the sun and you will see the differences .