I personally won't do any colour without a skin test, I have even turned business away because of this. A local hairdresser was sued by a client who only had a minor reaction, it's so not worth the risk.
If it's a foils, then flooded, you could always use a semi to flood.
If you want to take the a chance, you could carry out a risk assessment survey. A lot of colour companies are looking at bringing this in. This is so you can eliminate any risk of a reaction or likely reaction to permanent colour. I'm not sure if you would be covered on your insurance should the client still react.
You could also get colour start in. You can post out these skin tests to clients.
Trichocare