A. If you want to be a FULL TIME cabbie (taxi driver), then you have to paint or use yellow tape/sticker sheet to look your car as a cab. Then go to motor vehicle reg and tell them you're gonna convert your car into a cab. They'll charge some legal and some MITHAI (because most probably you are not gonna install a taxi meter in it) to get your cab reg and legal to drive as a public taxi.
B. If you want to use it as PART TIME cab, then I think it is okay to use it as it is. If you want this thing, then it's better to get daily commuters and charge them on monthly bases. i.e. pick and drop service.
It varies city-to-city, our greater Islamabad-Rawalpindi is flooded with "unmarked cabs"!!!
Yeah, it's very common in the twin cities, things are quite different in Karachi and Lahore.