Welcome to Librarium Online!
Before I place the sheild on I would like to paint the rest of the bike but cant think of a good color for them.
Record with my Necrons
W:2 D:0 L:0
2st game: vs Chaos result: Win :p
Sheild? Are you talking about the Vyper, if so I don't really remeber there being a motor. As for Jetbikes well I tend to just leave them whatever colour I've painted the rest of the jetbike, for example in Ulthwe (uses Black as a main colour and Bleached Bone as a Secondary) I would keep them black and highlight them.
Some people like having silver to indicate metallic objects but I think thats steriotypical for some of the more bulky and clanky races like the Imperium and Orks.
There isn't a right or wrong way so I guess you should just do whatever you feel comfortable with.