Color Quantification Help

Hi everyone, this might sound a bit unusual, but I’m working on a research project where I need to measure the total area of two different gradient-like pigments on leaves using Photoshop. I’ve tried selecting color ranges, using in-image rulers, analyzing histograms, and other approaches, but I’m still unsure as to of how to get "good" measurments. I’ve attached an example to show what I mean by pigment differences, the green vs. purple areas are what I’m trying to measure. I would really appreciate any guidance or suggestions on how to accurately quantify these areas.

Thank you so much for your help!