When I did my spray tan training over 5 years ago with the original Tantrick owners (the lovely Mandy and Stu) I was told that the original Tantrick solutions would never show a green 'ring' when tested on couch roll like you would get with many other brands that had 'oxidised' as it has a red spectrum in the tan and I found this to be so true, even with bottles that were a couple of years old, they never looked like they had 'turned'. I think the solution has been changed of late as I opened a new bottle of 12% the other day and it had totally oxidised and to me was unfit to use on a client, very dissapointing really.
Anyway,as for the new 'intellegent' liquids that are sold by the new owners of Tantrick ie Autume Gold and Essence of Summer, these have a green spectrum. My one and only bottle of Essence of Summer was totally green from word go but I was assured that this is because of the green spectrum and was totally fine to use. It put me off using it actually but many geeks on here rave about it. I do like the Autume Gold and this again has a green tinge after a while but I have sprayed reluctantly with this when its looking green and the client has not complained, so I think its quite ok to use when its green. I am not convinced but as long as I don't get any complaints then I will use up my stock.
If your client is quite dark skinned then I would go for the 12%, its not that dark really and should be fine on a darker skinned client. If your client isn't really looking for a darker tan, then use your 10% on them, it will still be fairly darkish on a darker skinned base.
HTH