Changing roughness within micro-tiled textures

hello all,
This is a very simple problem, I only got to work with unreal for a couple of days now. I have made a grass function with macro and micro variation the standard way, which adds a yellowish tone to some parts so it looks like dry grass. Now I want these particular parts to have a much higher roughness, to look dry. Also I’d like that as a parameter. I’m probably struggling because I did not fully understand the output of alpha channels in textures and the UV inputs yet(I know UV kinda describes the coordinates of textures and what alpha in the lerp function does, but not in e.g. texture samples). In the documentation these words are used to describe the bigger picture and one can kind of assume what it’s for, but it’s never really explained.
I’d really appreciate anything that could help me, thank you so much in advance!