Re: Need to convert to gray: Yes, of course you need to convert to gray for GLCM. I was simply saying that if your GLCM code -appeared- to be generating results that reflected the health of the leaves, that GLCM was keying on some textural quality rather than color. That’s a good thing.
But this definitely does not preclude the need for color analysis. That will be your most valuable tool for the initial process.
I suggest that you try to eliminate all pixels that do not tell you anything about the health of your plants. In other words, anything in the background, like pipes, etc. In the images that you’ve shown, these show up as flatter areas with colors that are distinct from your plants. You could turn all background pixels to black (0, 0, 0). Then do a histogram on the remaining pixels to find out the color spectra. This should give you valuable info. Your color histograms of healthy plants should give sharp spikes in green spectra. Less healthy would be more brown. That should be simple.
Then use the same image (with blacked-out background pixels) to generate your gray-scale image to send to the GLCM function. The combination of the color histogram + GLCM should get you part way to where you want to go.