Procedure to derive the BSS fraction. m_F814W vs. m_F475W − m_F814W CMD of stars in the cluster region of NGC 2213 (left panel) and in the field region (right panel). The pink-shaded areas mark the CMD region A1, which is mostly populated by MS stars, while the stars in the CMD region B1 which hosts the candidate BSSs are marked with azure dots. The azure solid and dotted lines separate the B1 region from the remaining CMD area. See the text for details.