The bird shop by me has just received four young yellows. I went to see them yesterday to see if there was a female. I was stumped

One has a light purple breast but couldn't tell the head color. The others are white breast and just starting to get red head feathers. I forgot to look for differences in belly color. They all have red tip beaks. Is there a difference in beak color for hens or cock that are yellow bodied?
I'm going back today and look again. Usally when I whistle I can get the males to sing, but yesterday they were still to excited. If anyone has any tips for telling the difference in youong yellows...Help
