How can I use image processing to track the progression of a certain coloured area in several continuous frames extracted from a video?

My goal is to track the progression of the grey area in the following set of frames, as it represents the movement of a micrometer whose speed I'm trying to control. I asked a question on this forum a few weeks ago (https://www.mathworks.com/matlabcentral/answers/442849-how-do-i-subtract-the-following-two-images-and-determine-the-number-of-pixels-in-a-certain-section-o) asking to determine the number of pixels in the grey area in one of the frames. While I found what I was looking for at the time, I now want to be able to compare any one of the frames to the initial/background frame, perhaps by some kind of image subtraction technique? Does anyone have any tips for achieving this?

Furthermore, the initial code I received uses a thresholding technique specific to the image I provided. Is it possible to write a code that determines and adjusts the threshold values for different contact fronts of various hues/saturations? (but still segments them into binary colours).

Please let me know, thank you.