The book is Panoplia Clericalis, and the passage I'm having difficulty with (which I suspect is much easier than I think) is, from page 602:
De colorum mixtione, qui differunt, ex varia eorum commixtione omnis ferè generis colores fieri posse existimant.
Which looks a bit like:
As far as a mixture of colors which are different, they(?) consider that from varying mixtures colors of nearly every kind can be made.
I'm not sure who "they" are, though. I'm wondering if I could translate that by something like "they say" rather than "they consider"; the next sentence begins:
Ex caeruleo enim et flavo (aiunt) fit viridis...
For from blue and yellow, they say, green can be made...
I'm imagining that "existimant" is intended to give much the same idea.
I had been questioning the occurrence of "omnis" as well but for some reason the fact that it can be a genitive form (and thus go with "generis") had slipped my mind.