Depends on the tank and stock too. In my 180g with the tangs and lots of other fish my skimmer is full of a thick almost motor oil kinda consistency. It reeks to high heaven it glops out when I clean it. Totally gross lol
On my 90g and other tanks its a dark brown usually, liquid no doubt but a nasty black liquid.
You can adjust the type of skimmate you get by adjusting the skimmer. Just depends on the skimmer as to how to adjust it
180g- Mostly BIG fish and some coral. ~80g Nuvo- My coral tank with "happy fish"~ 90g- FOWLR Not the not happy type of fish~ 125g- Freshwater Malawi Cichlids ~10g- Nuvo- The refugees from the Ich of '18