Welcome, Jaypee! :wave:
Specifically, titanium dioxide.
Like Shari said^^^ it depends what kind of titanium dioxide it is. There are a few kinds- either water dispersible, oil dispersible, or the kind that disperses well in either water or oil. For what it's worth, you can disperse any of them in glycerin if you are not sure what kind you have (i.e., glycerin works well with all of them).
Yes- from what I understand, micas are mined from silicate minerals (or can be created in the lab), and oxides naturally come from metals such as iron (although they too might be able to be created in a lab- though I'm not 100% sure. Will have to check on that). Anyway, I've found that it normally takes much more mica to get a good color in my soap in comparison to oxides.
I'm not sure whether I'm understanding your question correctly in regards to seeing lots of wastage. Did you mean to ask why would someone pre-mix all their colorants (in bulk squirt bottles), or why would someone
not pre-mix all their colorants (in bulk squirt bottles), i.e. mix them fresh from powdered for each batch?
If the latter, I can answer that question quite easily since I don't pre-mix any of my colorants in squirt bottles. I mix them fresh from powdered for each batch. And because I learned through trial and error (whilst taking lots of notes) in my beginning years of soap-making how much of each powdered colorant worked in my batches to my liking, there's no wastage at all.
Charcoal is derived from heating/charring plant material, and as I stated above, mica in its natural state is from silicate minerals mined from rock, but can also be created in the lab.
IrishLass