You can adjust the type of skimmate on certain skimmers depending on what you want. For example, on the AquaC Remora, you get lighter, more watery skimmate if the further down the collection cup is set.
As mentioned, different tanks and different skimmers will produce different kinds of skimmate. But generally speaking, it should have at least some colour and bad smell since the skimmer should be removing decomposing organic waste.