For some reason, with or without any resource packs, on both OptiFine and Iris, some textures such as grass, leaves and paths appear gray or discolored. It doesn't happen with other shaders, and it also doesn't happen with beta 1.15 version of your shader as well. I tried reinstalling everything to check if it's something on my side, but the problem's still there.

not a bug. that is the "seasons" feature.
you can configure it in shader settings -> climate -> seasons