The most common species in the Atlantic Forest is Violaceous Euphonia. The female (Fig. 1 & 2) is uniform olive-green above and below with dark grey inner webs to the primaries, tertials, medium and greater coverts. This is the only euphonia in this group with no grey on the underparts (note that female Golden-rumped also has olive-green underparts, but is easily distinguished - see here).
Perhaps the most distinctive is female Chestnut-bellied Euphonia (Fig. 3 & 4). The upperparts, flanks, ear coverts, chin and forehead are a rich yellow-green and it is the only species with red undertail coverts and vent. The grey center to the breast and upper belly extends onto the neck sides to form a half collar that is separated from the grey nape by green ear coverts.
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Female Purple-throated Euphonia (Fig. 5 & 6) is also distinctive, once you get your eye in. The general jizz is of a paler but more contrasty bird, with the colours on the upperparts often looking rather washed out. The upperparts are mainly pale olive-green becoming greyer on the mantle, nape and crown, and contrast sharply with the bright lemon yellow flanks, vent and undertail coverts. The breast and belly are pale grey (sometimes almost white) which set off the dark grey legs. The head pattern is also rather distinctive, with dark grey or black lores (meeting across the top of the bill) and faint narrow black eyestripe behind the eye, contrasting with a bright yellow forehead and hint of a yellow supercilium before the eye. The bill is a little longer and more conical than the other species, with a blackish distal half (more so on the upper mandible), and at close range the split white eye-ring is diagnostic. The blackish upper tail, and inner webs to the tertials and inner primaries add further contrast to the upperparts.
The two most similar of the set are Orange-bellied Euphonia and Green-chinned Euphonia. The females of both are olive-green above, pale grey below with olive-yellow flanks extending onto the undertail coverts. Orange-bellied Euphonia (Fig. 7 & 8) is polymorphic, but the form in the Atlantic Forest has a grey nape that stretches around the neck forming a very diffuse grey collar. In comparison, Green-chinned (Fig. 9 & 10) shows obvious grey neck sides extending onto the ear coverts, an olive-green nape and dark grey lores. Structurally, Green-chinned is a heavier looking bird with a noticeably thick bill, compared to Orange-bellied that has a small and rather delicate looking bill.
Many thanks to Leonardo Pimentel for permission to use his photos of Green-chinned Euphonia.