In my view, only the naked eye can do this job -- sensors will prove unreliable because the entire white frame (the metaphorical "search screen") has to be scanned to determine whether one or two colors "appear" in the window (in addition to other method constraints). The loop to be removed follows this scanning.
I would recommend that teams attempting this mission be prepared to get an instant judgment from the referee in attendance as to which color slabs are in the window before attempting loop removal. Even more important, tell the ref before the start of the match that you may ask for an instantaneous call -- I wouldn't want anyone to be penalized from an end-of-match difference between the team and ref as to whether one or both slabs are/were visible in the window. A referee who knows what the team may ask of him may be better prepared to respond/answer.
Finally, putting in some time between the slider push and the loop yank gives more time to get this ref feedback.
Don't forget Benefit of the Doubt (Rule 3) in close calls. This is for the team, not the coach, though it's perfectly legal to burst into the strains of Auld Lang Syne if that is your team signal :-) Just kidding!
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