What are the colored borders on shots for?

Sometime in the last year or so, after and update, I have found the colored borders on the shots.  Not sure what the purpose is, and has caused some strange functionality of preview shot changing when clearing a layer above.  Example, in layer 3 we keep most of our camera angle shots, layer 2 we have overlays for desktop presenter shots ( song lyrics etc.  from another computer running Pro Presenter) .  I am using X-keys 128, and when clearing layer 2 desktop presenter, my preview shot changes, I think to the green border shot.  I generally want the to keep whatever shot I have in the preview window, but when clearing layer 2, it changes.  When I go live after clearing layer 2, it clears layer 2 in the live shot OK, but I end up with a different shot in my preview window, changing to the camera preset as well.  Again this all changed with an update, not sure when though - certainly a number of months ago.  I liked it better before it did this.  I have reset preferences, and had to start a new project because I lost the audio meters, and other crash issues.  Looking through the manual I can't find anything regarding those colored borders, and what function they serve.   What does it all mean?????