Starlings

Please, Please! Could someone body help me? Like everyone else who feed garden birds I get tone of Starlings come to feed which is not a problem I have. What I like to find out is why do some of the Starlings have dark beaks and some have nearly yellow beaks?

All I can put it down to is some are young birds and some are adult birds. Am I right in thinking?

My Flickr. Photos link HERE