Here is a bit of information from www.arkive.org/common-tern/sterna-hirundo/
"...The bill is orange-red with a black tip and the legs are red (2) (3) (4) (5) (6).
Outside of the breeding season, the common tern retains some of its characteristic black cap, but the forehead and face become white. The bill and legs also lose their orange-red colouration and turn black (2) (5) (6). The male and female common terns are similar in appearance year-round (2).
The juvenile common tern is variable in appearance. In general, it has a whitish or buffy forehead which contrasts with a dark brown crown and dark ear-coverts. The back, shoulders and wings are most often grey, with prominent brown bars and dark edges and tips to the feathers. The underparts are white and the tail is short, lacking the elongated streamers, with dark tips. The legs of the juvenile common tern are pinkish or orange-brown, and the bill is dark with a pinkish or yellowish base that becomes darker with age..."