How to analyze and segment reflectance, absorbance, proportion of flowers

I am analyzing three images of the same flower/inflorescence, in which I took a visible, IR, and UV photo. I’m new to using ImageJ and I’m having a hard time determining how to analyze my images. I need to assess (1) the strength of reflectance/absorbance of an inflorescence in the UV spectrum (1st photo, taken with a UV filter), (2) the area (or proportion) of dark bracts or light (2nd photo- flower in the visible images, and (3) the temperature (i.e. absorbance) in the infra-red images (3rd image).

I’ve tried using segmentation to analyze the inflorescence using the SIOX plugin, which partially works, but I’m unsure if that is the best way to analyze the inflorescence considering how inaccurate it can be on the edges of the inflorescence. I’ve also tried using the micaToolbox plugin, which I’ve had trouble getting to work on my Mac.

Basically, I’m first trying to figure out how to calibrate the images (I used PTFE for gray standard in the IR and UV photos), then decide if I should analyze the whole inflorescence or just a portion of it, and lastly analyzing the photos in the three ways described above.

If anyone has any suggestions, I would greatly appreciate it!