I've mixed model paints before and its just been a case of make sure you're mixing like paints so they don't react in some way you weren't expecting so one option is to get a small quantity of dark grey or black paint of the same kind and blend till you get the desired result. Mixing well is the best bit of advice I can give you don't want it to start changing shade half way as you found an untouched pocket of white.
The other thing to factor in is make enough for the job before you start. Its something you mixed and unless you do very precise 1 part this for 10 parts that you won't hit the same blend twice.
Could be worth talking to the person on the mixing counter at BnQ as their machine does the measures accuratly but if they can tell you where to get the dye that could be an option too.
Kind regards
Ant