Saw this Niltava today at TPK. This first year bird clearly has a black lower mandible. Although I didn't get a photo of the underparts, in the binoculars the undertail coverts and belly were lighter than breast but still orangish somewhat similar to King's Park bird (which I went to see yesterday).
As a comparison this bird was the Niltava I saw 23 January 2013 also at TPK. It is an adult and clearly a different bird but it also has an all dark bill.
I think all these birds are probably Fujian Niltava and the pale bill perhaps isn't that usefull a fieldmark. Of course, I don't know for sure and there aren't a lot of photos of Fujian Niltava available on the internet.
