Just curious what the paint marks on the driveshaft of a 68 represent?
The identification colors helped line workers pick out the correct driveline for your cars specific application (example 302 automatic Cougar) from all the others stored and available at that station along the assembly line. These colors were also listed on the buildsheets tapped to the front and rear of the moving body by the time it reached that (the driveline installation) station. The workers could look at the sheet and confirm which driveline to choose from the rack.
At most plants most years the driveline was placed with other items to be installed and the unibody was lowered from an elevated section on the line to a lower one.
Often there are additional marks and stripes applied to some drivelines, some years and applications. We don’t know what each communicated, confirmed or were placed there for right now.
Here are a couple of examples of markings and the related color “call outs” on the buildsheet. The third from the last box (#27)in the section is a missing “W” from damage to the sheet
“Driveline” refers to all the parts of the engine / transmission / rear axle as an assembly. Driveshaft is (I think) what Jeff means. If you look at any build sheet or Master Parts Catalog the word is Driveshaft.
We have a chart of Cougar driveshaft colors for 1968 - 69 that predates the internet. Some of the paint codes come from looking at multiple driveshaft examples. Others came from build sheets. At this point we have build sheet examples that verify any and all of the ones on this chart.
Of interest is that the driveshaft was beefed up in April 1968 for big block applications (X code, S code, W code, and R code all shared the same driveshaft). The driveshaft became 4" diameter at the center while still using 3" diameter at each end. We assume the beef up is due to the new 428CJ engine, but it is not documented to be caused by that. The interesting thing is that the paint colors were unchanged from the previous version that was 3" diameter from end to end.
Yes the term driveline for what some call driveshaft 
In my region of the common term for them. Lots of names for parts of car differ depending on where you grew up or what you were taught. Kind of like the term third member, pumpkin, pig, and so on for the center removable section of a Ford or Chevy rearend . Around here the engine, transmission and rearend as an assembly is referred to as drivetrain.
Just as long as the information gets communicated. Thanks for mentioning driveshaft and making it clear for the OP.
Thanks for the information