I have to search this but I did speak to an engineer about lubricants. He said that lubricants have detergents in them and over time it wears out and that too shows up as a change in color. Its not only a dirty chain but also just the way detergents wear out.

Once you use a chain cleaner like Park Tools, or something like that, the cleaning fluid shows residue and that's not the detergent. So its both.