Color‐color diagrams. (a) U−B vs. B−V diagram for V ≤ 17 mag. The thin and thick solid lines represent the intrinsic ZAMS relation and reddened ZAMS relation, respectively. (b) B−V vs. V−I diagram for V ≤ 18 mag. The thin dashed line indicates the intrinsic relation for giants. The other two lines are the same as (a). All symbols are the same as in Fig. 2.