Let's take a look at these immature plumages. Few field guides illustrate them, although the Howell & Webb guide to the region does routinely describe them, if not in very much detail. Generally, younger birds are characterized by buffy feather tips on the upper and sometimes under parts. In the hand, presence of bill striations (fine corrugations along the side of the maxilla) indicates a younger individual. (Double clicking on these photos will reveal them in some cases.)
The top bird is a young Green Violetear. It will lose those buffy tips on the head and back through a first prebasic molt during its first life year. Those dusky underparts will become metallic green.
Both sexes of Green-breasted Mountain-gem show dusky greenish throat feathers, which they quickly replace with (usually buff-tinged) whitish feathers in the case of the female, or bright green feathers with white borders in the case of the male.
Here's a female. Note the buffy tips on the head and back, and the dark throat feathers, both characters associated with immatures in this species.