I just wanted to point out that
mandellii can actually be quite bright with quite strong green and yellow tones. This seems to be the case with fresh birds in autumn. Examples of how bright they can be can be seen in Ayuwat's excellent images from Thailand here:
https://ebird.org/media/catalog?tax...=Thailand (TH)®ionCode=TH&q=Hume's Warbler
There' s quite a contrast between those birds, which astonished me when I first saw them, and, say, Wichyanan's bird in row 3.
There are some more subtle differences between
mandellii and Yellow-browed. Firstly, on
mandellii, only the base of the lower mandible is flesh-coloured. The lower mandible on the OP bird is pretty much entirely yellow with just the hint of a dark tip. Also in
mandellii, the upper (median covert) wing bar tends to be weaker, the dark centres to the greater coverts are less pronounced and the dark primary/secondary bases (forming the dark lower border to the main wingbar) are narrower and weaker. The reason this feature is absent altogether on the OP is that, as John pointed out, it's in moult.
I'm still puzzled by your statement, taurus1971, that
mandellii passes through Korea. I've never heard anything about this before. Any more references?