Being red can mean capacity at the exchange can cause higher contention than normal and thus lower speeds. Until it goes green that is the most likely cause and there is very little you can do until that occurs. If it is still slow when Green, then you need to investigate further.
You could try the BT Speed tester. If that shows normal speeds then it is a PlusNet problem rather than BT and you need to report it.
My exchange has been on amber and mostly red for ages now and now my connection is just pants. I have 1mb and right now have connection of 507 (ADSL guide) this was after reboot of router prior to this I had the dizzying height of 567 on plusnet speed test. Does RED mean this massive drop in performance for me till BT get things sorted? Used to get regular 937- 950 prior to these troubles. problems seem to worse during the week and slightly better at weekends.