Normally if you're turning right you should enter the yellow box. The only time you are not allowed to enter them if your exit road isn't clear, by which I mean the road you're turning into should be empty -- it doesn't matter if the junction itself is clear or not, only your exit road. So if you're being held up by oncoming traffic (as is often the case) then it's fine to wait in the yellow box until the traffic has gone past.