Hi,
I have been using Sienna X for a while now and carry with me (I'm mobile) 8, 10 and 12.
Generally I use the 8 or 10 depending on the clients colouring and the depth they want to achieve.
I've used the 12, but only once. It was on a client that had an olive skin tone, I was a little nervous to how it would develop... She'd had the 10 perviously but wanted to be darker. The 12 developed lovely and she was very happy.
Sometimes I'm unsure on deciding which to use (especially on a new client) so I always go with the lighter option as I'd rather they be too light than too dark.
I hope this makes sence and helps!
Suzy x