For a 3 pound package of bees, my feeling is that an entrance reduced down to 1” is sufficient do deal with most robbing. The idea of the small entrance is it slows down how fast the robbers can get into and out of your hive. Like the mall entrance at Christmas, if they only open one door, people can’t get in or out very quickly. Small entrances slow the robbers down and limits how much they can steal, but it also slows your girls down too. Like most things with bee keeping, it’s a compromise. Personally I don’t like robber screens, I use reduced entrances when I need to. I have no robbing issues right now in any of my hives.
The real problem is our recent warm spell and your 1:1 syrup. The bees really smell that as the temps raise. It looks like we’re going to cool down a bit for the next week so I would monitor the hives and you may see the robbing subside to some degree.
To really get rid of the robbers you need to get rid of the 1:1 syrup. I’m not running packages here, but I have caught 5 swarms this spring and only gave them 1 medium frame of honey for food. There seems to be plenty of nectar in MI right now even though dandelions are starting to phase out. If your bees have a frame of stores built up, and the robbing is out of control, then I would probably stop the 1:1 feeding and let the bees collect nectar from outside. If they have nothing built up, then you probably have to accept some robbing for a while.
I have Italians and Carnioleans. The Carnioleans are a darker bee and that is probably what is robbing your hive. It is true that older Italians do start to look darker, but the Carnioleans are probably what you are seeing.