A singer's tessitura depends on where their voice sounds the strongest (and most comfortable);
Sopranos and tenors sound the strongest and most at ease in their high notes but weaker in their low notes;
Mezzo-sopranos and baritones sound weaker and less comfortable in their high notes but strongest and most comfortable in mid-range and strong in their low notes;
Contraltos and basses sound the strongest and most comfortable in their low notes and weaker in their high notes.
Sopranos and tenors generally sound bright, contraltos and basses generally sound dark, and mezzo-sopranos and baritones are of medium color. But a singer can sound as bright and sunny as a soprano and have a contralto tessitura or another can sound like a baritone yet have the tessitura of a tenor.
Sarah Brightman has a bright, lightweight voice and the tessitura of a soprano.
Allison Crowe has a heavy, rather dark voice and the tessitura of a soprano.
Idina Menzel's voice is lightweight, has a bright color and the tessitura of a mezzo-soprano.
Annie Lennox has a dark, heavy voice and a contralto tessitura.