How I can determinate an action for a rage RGB?

Hi everyone,

I'm working with image processing for microscopy photos, the idea is to process them with a Threshold from the app, which I already did, but since every image is different, regarding backgrownds, I need to adjust the script to have an "if" function to determinate which Threshold function use. My idea is to use ranges of RGB to cataloge the photos and then make the script to define which function to use (which are 4 diffferent options).

can someone help me, which function or comand I can use to define a range and then make it work on the "if" fuction?