Both. Clean oil retains its colour visually. Have a look at the thread re oil analysis for an example of a typical report. Oil analysis is useful not just for the state of the oil but for pending problems - eg, dirt due to a leak downstream from the air filter, overfuelling, coolant (pending gasket failure, etc. Also gives the wear metals and you can see the compositions of each change eg, during break-in on a rebuilt motor versus a fully run-in one.