AFAIK Vehicle Crew (bar Landspeeders and Bikes) aren't drawn from companies...
If you look in the vanilla codex, I believe it shows the vehicle pool as a seperate entity from the rest of the Chapter, they are simply assigned to a company for the duration of an operation.
Vehicle crews are (I THINK) supposed to be painted like adepts of the machine god, hence the cog symbols... So paint them like you would techmarines, a lot of people don't do this, though... But I'd say just give them a dark red helmet and dark red shoulderpad/s depending on how many pads have a cog symbol
Check Lexicanum, it has this data... Apparently a Rhino only has one crew member... not sure I'd believe that, I'd assume that's when it's configured to its default setup.
The Honour Guard are a seperate pool, I think... at least, that's how it was implied to work in Chaos Rising. I believe they must be drawn from the 1st Company originally, but I don't think they're within its organisation.
Automatically Appended Next Post: Oh I forgot to add that Landspeeders are crewed by Assault Marines from the Company they're with, I believe.