question for colorists out there...

Hi,

I posted this before, but didn`t get any advice on whether there are any rules for colouring.

I want to mix a medium and a dark color,
and I want to mix a neutral and an ash.

Q: are there any rules for whether the neutral should be the dark one or the medium one? (or can I take my pick of a)dark neutral/medium ash and b)dark ash/medium neutral).

Please don`t tell me to get it done professionally - I am a student on mat leave and CANNOT afford that... not for a long, long time....

Thanks in advance,
Hannah