So bags are more of an over-your-abaya item than under-your-abaya item, but where there goes an abaya there goes a bag. On the rare ocassion I have dared to venture out of my house (I wear an abaya full-time) without a bag, I have felt naked.
And so you may wear an abaya, but that doesn't mean your bag shouldn't co-ordinate with the abaya you are wearing - you wouldn't wear a glamorous evening clutch with your everyday, plain work abaya, just like you wouldn't choose a casual hobo or bucket bag to wear with one of your more special, embellished and well embroidered abayas.
I think the whole acid bright/neon coloured bags are great - choose a minimalistic type abaya and pair it with one of these bags and the contrast will be amazing! Like I said, that's colour blocking to the max!
The important thing to remember: the right abaya + the right bag = glamour, glamour, glamour.