I started out with plastic Easter Eggs, filled with sand and glued shut. I wanted eggs that were NOTHING like what I would get from any of my pullets... because I didn't want to fake MYSELF out.
Bright yellow, bright red and vibrant pink eggs.
There was an egg eater in the flock, and she managed to get one or two plastic eggs out of the coops entirely, and then broken open.
So I scavenged up some golf balls. One of which is yellow. Two others have writing on them (given as gifts for my dogs, who don't like playing with golf balls) and I try to keep one in each nest.
Sure is amazing how they get moved around from nest to nest. Periodically I redistribute them, one to each nest, again.
The ONLY nest boxes I do not have them in are the built-in rollaway nest boxes, but four of my eight current layers use those nest boxes anyway.