paint their toe nails a different color - my friend said when they were children they use to paint the chicken toe nails and it sounded funny! And the girls could look really pretty!! I thought about putting a dot of paint or indelible marker on their heads.
While it may be cute, it is fairly impractical as the longevity of nail polish on the bird's nails will be such that it will be a matter of near constant re-application and the nails are not very visible in a "flock observation" situation so you still end up with a group of birds unable to be told apart without very close inspection of the nails.