\begin{sequencediagram}
  \newthread[red]{r}{:Red}
  \newthread[green]{g}{:Green}
  \newthread[blue]{b}{:Blue}
  \tikzstyle{inststyle}+=[top color=yellow, bottom color=gray]
  \newinst{y}{:Yellow}
  \tikzstyle{inststyle}+=[bottom color=white, top color=white, rounded corners=3mm]
  \newinst{o}{:Rounded}
\end{sequencediagram}