In my watch window, some of the items are in red, and some are black.

But I can't see what this difference in colour indicates.

Does anyone know? Or can you tell me where it is in the documentation?

Red means the value of that item (or one of its underlying items if it is an array/object) changed in the previous execution step.

That execution step could be either a single statement if you are single-stepping, or a lot of statements if stepping over functions or running between breakpoints. The debugger compares the watch values each time it pauses execution and shows the changed values in red.

Thanks for that - I couldn't find it in the documents, but that clarifies.