Colorization - what was it's purpose?

[BessAgritianin]

But all those pigments are naturally-occurring minerals that don't need any "alchemical" processing other than grinding. 

And anyway,alchemists would have been unable to make azurite or orpiment.  Even today they require special lab equipment.

That is simply  not correct. Medieval manuscripts contain entire chapters on chemical creation of paints- gilding, silvering and pigment manufacture.
Me personally- listing over some alchemical works have found (usually at the end ) descriptions for creating of different paints and o yes they knew quite well  orpiment manufacturing- not only grinding of material but using furnaces, crucibles and distillation.
Check this You are not allowed to view links. Register or Login to view.
 You do not need to go far but see the "pharma" part of VM.
Actually there are a lot of sources for medieval paint manufacturing, but this is another theme- and since there is no "Alchemy/pharmacy" department in this blog, we better leave it fornow.
Here is important to say that the fact that Voynich contains this pigments is another evidence for its authenticity.