The red bill and ring around the eye, as well as the white markings certainly resemble a Livingstone's. The white "tear" marking doesn't really match though. I actually can't find any turaco with the marking going down
Singapore has never had Knysna Turaco as far as records go (at least on Zootierliste).
This is definitely a White-cheeked x a Tauraco sp.
I am 99% sure that Tuaraco sp. is Livingstone's.
The reason for this is due to all the Singapore facilities only having Livingstone and Schalows turaco that would match the characteristics of a green turaco with a white stripe at the edge of its crest.
If it were to be a Schalow's Turaco (much rare in Singapore than the almost omnipresent Livingstone's Turaco) then the crest on the Hybrid would have been longer almost representing that of a normal Livingstone's Turaco.
Therefore the turaco you are looking at is most likely Livingstone's x White-cheeked Turaco
(Tauraco livingstonii x Menelikornis leucotis) [previously Tauraco leucotis]