I have found that there are two circuit board colors for the instrument cluster . Green and Red. Question: Are they the same or is there a difference? And what would the difference be ?

 

Not sure if the color is significant, but there are differences in circuit boards, with or without gauges, location of the brake warning and hi beam jewels, in '75 the "low fuel economy" light. All of these took different pc boards.
There should be a Ford engineering number on the board, that would tell you which one it is.

 

hey mike, do you know what exactly controls the low fuel economy light on the 75

 

Yup, this vacuum actuated switch that's mounted on the firewall.