Since no one else is addressing this, I'll give it a try. Oil sampling should be done just prior to a scheduled oil change. To be meaningful, it should be done at each oil change. That way you have a progressible history of the normal wear metals in your oil. As long as samples show near the same amounts of wear metals, all is well. When one or many metal counts shoot up, you have a problem. With oil analysis you have a warning, usually, well in advance of a failure. One thing to be ware of, however, is that changing oil brands (or types) can often change the wear metal readings on your analysis. Many of the oil additives are organo metallic compounds and will show up as wear metals on the plasma spectrophotometers commonly used by labs. Always have a clean oil sample run to get a base reading of the clean oil metal content.
Darkening of oil can be many things. Assuming you have a diesel, darkening is normal and is usually soot. Good diesel oil will keep the soot in suspension. The filter will get some but most soot particles are too small for a filter. Lots of soot will thicken the oil over time but proper oil change intervals should check this condition. Oil analysis will help you with determining a proper oil change interval under unusually high soot conditions. Some diesels will soot up the oil very quickly, but thickening is unusual during the recommended change interval. You might have combustion blow by problems which would be accompanied by hard starting if the rings were starting to stick. Also, high idle time is good soot producer. Don't idle more than a few minutes. Shut it off. Hope this helps.