Bridges should show red by default during opening hours, then flashing red when the bridge keeper knows you are there, then green for go.
I have noticed recently that Rea and Sellars (two of the higher ones, don't need to swing for narrowboats generally) appear to be unmanned this season, so may not show lights at all. I've not seen this information communicated anywhere, so it does cause confusion.
Generally the bridge keepers are pretty alert and they will usually keep in contact with each other so they know who is coming. If they don't see you, sound your horn. If they still don't see you, ring them (I have all the bridge tel numbers saved as contacts). I find it rare to have to use the horn and very very rare to have to ring. The latter is usually only needed when you have just left a mooring and so the keeper does not know you are coming.
Note that Sandfield is remotely operated from Fretherne. Off season, Cambridge is operated from Patch (but the keeper has to drive or cycle between them, so you must ring).
Rare to encounter a grumpy bridge keeper - most are very cheerful and friendly - but it does happen sometimes...