The Green square is the Middle C (C4) and the Blue square is the High C (C6), which is the typical vocal range for a soprano. The Red squares show Callas's vocal range in performance, going from F-sharp (F#3) below Middle C to E-natural (E6) above High C.
<a href="//commons.wikimedia.org/wiki/User:Penyulap" title="User:Penyulap">Penyulap</a> <a href="//commons.wikimedia.org/wiki/User:Amit6" title="User:Amit6">Amit6</a>
<span class="int-own-work" lang="en">Own work</span>