Most varities of parrotlets are sexually dimorphic. In general, the females have less blue, and more yellow, than the males. Green rumps, like Peanut, follow this rule of thumb: If Peanut were female, the area between and around his eyes would be yellow/green, like a Granny Smith apple. This photo
(from Shady Pines Aviary
) shows the color differences between male female green rumps: the female is on the right.