Hi Kirsty, the reason is that they have used artificial colours in the tan. In order to get a brown colour, they would have used red, blue and yellow. However, the red degrades when it is exposed to extremes of temperatures so all you are left with is blue and yellow = green.
Go for a tan that does not contain these artificial colours:
Red 40 (CI 16035)
Blue 1 (CI 42090)
Yellow 5 (CI 19140).
Just for info for anyone finding their bottle of tan has gone green! -
I had the end of a bottle of tan go green last week. Fortunately it was the Fast Tan that I only use on my daughter. (I just wanted to use it up!) - it was ok the week before, so it went quite quickly! I only really noticed it as I was spraying her and as it was my daughter we decided to carry on as we were both curious to see if it actually would take still! (If it was on a client I definitely wouldn't!!) ... anyway, the result (even after sleeping in it) was much lighter than normal. I suspect it'll wear off faster too, so it would appear it does effect the quality of the tan. (I will add this bottle was opened over 6 months ago though, so not sure if it was a newer bottle).