Rip the blindfold

Before they are blindfolded, three women are told that each one will
have either a red or a blue cross painted on her forehead. When the blindfolds
are removed, each woman is then supposed to raise her hand only if
she sees a red cross and to drop her hand when she figures out the color of
her own cross. Now, here’s what actually happens. The three women are
blindfolded and a red cross is drawn on each of their foreheads. The blindfolds
are removed. After looking at each other, the three women raise their
hands simultaneously. After a short time, one woman lowers her hand and
says, “My cross is red.” How did she figure it out?