I have, on occasion, done a colour without a patch test.
It is not something I would advocate though. As long as you check they have no allergies, have never had a henna tattoo and no skin conditions, you can be safe.
L'oreal were trying to get some sort of a legal document drawn up, so you can colour without skin testing but I don't know how far it has got. Most people are ok, so they argued it was a case of checking out any previous problems via questionnaire and using your common sense.
Patch testing is not always fail safe either. I once tested a customer 48 hrs before but they then went on to have a mild irritation after the colour, even though the patch test was negative.
Not many hairdressers patch test every colour, which is a recommendation every 6 weeks, as it is time consuming and often inconvenient to the client.
A tricky one I think.