Colorbond steel has a zinc/aluminium alloy mixed with it, so I am assuming that since you can see no rust, that the powder u observe is an aluminium oxide. This will stay around for decades and not shift if you leave it alone, but i can't comment on what effect this has on drinking water. Even the bare metal (steel) I think will still not corrode (rust) unless the alloy coating is too far removed from that spot. The aluminium/zinc will oxidise first before the iron in the steel.
So basically, unless you don't like the look, I would leave it alone.
Unless the paint has specific anti-oxidants in it (like anti-rust paint), then the steel will still oxidise at the same rate, pretty much, in outside conditions. might slow it down by a year or two, but rust will then start to show as the paint opens up hairline cracks over the bare steel. At that point it's no different to treating rust on a car. But, as I said above, it depends how large the
bare patches are. If you paint it be sure that the paint used is safe to drink from once it is dry. No nasty lead additives etc like in some Industrial paints, which made Beethoven deaf.
This link talks further about painting colobond
http://www.colorbond.com/go/home/residential/roofing/tips-and-advice/repair
Cheers,
JB
