Are they plastic or aluminum?
If plastic, go with Krylon Fusion or RustOleum Paint-for-Plastic. Both come in spray cans and are chemically formulated for plastics. I've used the latter on external vinyl shutters with great results; search around and you'll find a couple of guys who've used Fusion on the outside of their cases.
If they're aluminum, you should be able to use any good quality paint, but you'd really want to put some primer on first, I'd guess.
In either event, clean thoroughly, and use some rubbing alcohol at the last cleaning step to remove any oily/gummy stuff.
For a quick & dirty fix in rectangular cases you could try some self-adhesive shelf paper...
'07 DL650 ABS
'04 Yamaha V-Star 650 Classic Silverado (gone)
'04 Yamaha TTR225
'03 BMW F650GS (wife's)
'92 Honda XR250L
'86 Honda VT500c (gone)