I am 7 days into LR trial. After creating and 'using' Develop to edit 6 catalogues, I find without changing anything that all the catalogues when opened, now have a bright red highlight in different areas of the pictures within the catalogue. Some of the catalogues were created several days ago and have been viewed as OK, but now this 'red' tinge has appeared in all catalogues without any intervention by me.
I'm not sure why you are creating several different catalogs. Lightroom is managed much more efficiently by using a single catalog. But regardless of that issue, it sounds as if you have turned on the clipping indicators. Pressing the J key will turn both the highlight and shadow clipping indicators on or off. Or you can click on the little squares in the upper corners of the histogram. Clicking the square on the left will turn shadow clipping on/off. Clicking the square on the right will turn highlight clipping on/off. I leave the indicators turned on so that I can identify areas that need to be adjusted so that I can eliminate all clipping.