Andy I read an article on the gauge resistors and according to them the senders have certain dead spots in their resistance to heat to ground relationships. The sender will have a spot where the coolant heat has to rise quite a bit to effect any movement in the gauge needle.
To smooth that out, the engineers calibrated the resistors to the senders and the gauges and I assume the colors identify the resistance values.
one of our members, Tom TBone has done quite a bit of experimenting on the gauges and resistors and he may know more about the subject also he has a stock of resistors and he repairs gauges and clusters.
Here's a thread by him on fuel gauges so you can PM on the subject.
http://67-72chevytrucks.com/vboard/s...d.php?t=640615