Harry,
You dont have to identify the fractions in advance. It is fairly common in chromatography for fractions to be colorless which means you identify them after the fact, generally via analytical testing. In the case of using butane/propane, you’ll be able to evaluate the visually within a few hours based on color as well as precipitation of solids (thca, waxes etc). My goal is to pull fractions that are substantially different from each other, with minimal overlap of undesirables. I can always combine similar fractions later, or re-run muddy fractions to purify. I love chromatography because you can always get the results you want if you’re willing to put in the work. 