Scarlet-backed Flowerpecker Dicaeum cryentatum is the second commonest flowerpecker (after Orange-bellied Flowerpecker D. trigonostigma) in Borneo. The male shows unmistakable scarlet crown, back and rump. The female shows red rump. Nevertheless, a variety of color morph has been reported, including a singing male with black back in Tawau (Gretton 1990), a specimen from Banjarmasin, Kalimantan Selatan, has red crown and back, but is otherwise female in plumage. Hybridisation with Scarlet-headed Flowerpecker D. trochileum (which does not occur in Sabah) has been recorded near Samarinda, Kalimantan Timur.
Undoubtedly the field identification of juvenile birds can be nightmarish at times, unless you witness the adult birds together as well.
Description of the juvenile of the Scarlet-backed Flowerpecker by Robson's Field Guide to the Birds of Southeast Asia (2008) is that it is similar to female, but basal half of bill mostly reddish pink, underparts uniform with orange-tinged upper tail-converts, throat and breast more uniform greyish. Susan Myer (2009) says it is like female but lacks red on rump, underparts buffier, no mention of bill color though.
Here are pictures of the female and juvenile, the bill color of the juvenile is more yellow-orange rather than as described by Robson.
Female bird, note the red rump.
Nearer bird is juvenile, further bird is adult female.