J, J − K_s colour–magnitude diagrams of the Arches outer fields. Labels are as described in Fig. B.3, and the J, J − K_s diagram of the Arches central field is shown in Fig. 19. The variation in main-sequence colour between fields caused by the locally varying extinction is more pronounced than in the Quintuplet fields. Notably, several L′-excess sources do not show excess emission at K_s.