Well, the configuration makes sense if you have the top two panels:
DWN - FWD
Then the Green - sweet spot - in the middle
Then
AFT - UP
If the plane being refueled dives slightly it will also speed up slightly and go forward slightly (relative to the tanker), and if it climbs slightly it will also slow slightly and will go aft slightly. I presume the center Green or Blue panel indicates all is steady and centered in the refueling limits.
Yes, the green is the optimum refueling position or sweet spot. The up, dwn, fwd, and aft are directive indicators for the refueling pilot to follow in order to require the sweet spot. On the follow on jet tankers there is a "green apple" on the center of the boom. When half of the apple is covered by the boom shroud and there are no corrective indicators lit up on the belly, the pilot is in a perfect sweet spot.
Hard to tell in this b&w, but this is a good view of being in the sweet spot, although slightly offset.