Amazon are selling the silver, bronze, gold pack for £3.89 (with prime)
Have tried it, and it does work.
I would only do small detail thou, large areas can look a bit funny
Not heard of anyone using gold / silver but a very small pen (usually 0.1micron) is often used for eyes. Also sometimes they get used for black lining between colours.
I've painted a sanginary guard using the gold. gives a pretty decent colour.