Age doesn't matter, you can tag adults--the younger ones are just easier because it's easier to see what you're doing. I usually wait until they're fully feathered out, so your's sound about right.
I couldn't find any good pictures....
Do you still have the box? There's a pretty good picture...
You just have to look around.......Culter Supply has them in silver (aluminum).
As far as the colors go--I have purple on my birds and I can't see them at all once the birds are adults and the feathers are grown over them. I like the colors because when I'm trying to find the band in the...