Can anyone recommend a non-permanent hair colourant suitable for v allergic person?

I'd be so grateful if any fellow allergy-sufferers can advise. I've been using the same hair colourant product for years, one of the N&E "Non permanent" range that fades in 24 washes. It's one of the type with two bottles of liquid that have to be mixed together. It was just right for my shade (medium/darkish blonde), but unfortunately I seem to have developed a sensitivity to it. The last couple of times I used it, I got a blotchy red rash on my neck, which took weeks to settle down.

So I obviously need to find something gentler! I'm thinking I might just play safe for a bit and use one of the type that only last for 8 washes (just one bottle, nothing to mix together) as I'm guessing they'll be much gentler.

Anyone had a similar problem and found a good product, suitable for medium blonde hair with just a bit of grey to cover up?

I'm also wondering whether this type of sensitivity is a forever thing, or is there any chance it might settle down so I can go back to my beloved regular product after a while? I'm a bit scared to experiment with it now in case it all flares up again!

Huge thanks in advance if anyone can advise.