Well, you actually said it yourself already ;-)
Add temperature of the liquid, temp. of the air. Particles in the air and a whole bunch of other things that could influence the reaction.
The official color is actually darkblue.
If you know physics (I don't) then the machines will tell you that the darkpurple/almost black we are always looking for is actually a sort of very dark blue (which for anyone else is definitely a very dark purple).
aj