I keep a stack of old business cards and do the blotter test every 1,000 to 2,000 km, particularly on the L67 (will do the LIM gaskets soon, I promise).
As to the colour ?
I just sponsored (through work), a super advanced blotter test for analysing steam turbine oils for varnishing and sludging.
A sample of oil is drawn through a membrane, and then washed with toluene and petroleum spirits to remove the oil, and only leave the oxidation products.
This is then colour compared against a fresh membrane that has been washed in the same solvents, and the delta E value reflects the degree of oil degradation...darker is worser.
Thesis proved that a desktop scanner, and GIMP were nearly good enough to do a lab like analysis, some extra colour manipulation software makes it near perfect.
So yes, oil colour, and blotter tests can tell you about how the oil has degraded in service...and alert you to other stuff into the bargain.