Chocolate would be a warm ganache. 2 pounds of semi sweet choc chips to 1 qt of cream that has been brought to just below a boil. Or you can reduce it depending on the number of petit fours you're trying to make. Take off of heat, add chips, stir to smooth. Use immediately if it's a glaze. Let cool and thicken for a "frosting". Whip for a filling. Make sure your cakes are frozen/as crumbless as possible before enrobing. (Use a soft pastry brush)
For a 'white' glaze you could use 10x powdered sugar, mix with a little butter, warm water and flavoring of choice. It's thin and transluscent. A little fluid goes a long way with 10x.
I've tried white chocolate ganache but it never set up. Still looking for that perfect recipe. But I think the glaze you're looking for would be the thin "buttercream" type. There are other types that involve boiling sugar syrup and all but I don't know how elaborate you want to get.
April