Thank you Gretchen, it also looks like a Crimson Sunbird to me.
Here is a list of sunbirds of S'pore from wikipedia, both Purple-throated and Crimson are "Common".
Crimson sunbird (Aethopyga siparaja) - C/RB - parks, forests
Brown-throated sunbird (Anthreptes malacensis) - C/RB - parks
Plain sunbird (Anthreptes simplex) - VR/R - CCNR
Olive-backed sunbird (Cinnyris jugularis) - C/RB - parks
Copper-throated sunbird (Leptocoma calcostetha) - U/RB - BBNP, SBWR, P. Ubin
Purple-throated sunbird (Leptocoma sperata) - C/RB - BBNP, BTNR, CCNR, P. Ubin
Yellow-eared spiderhunter (Arachnothera chrysogenys) - VR/R - BTNR, CCNR, P. Tekong, P. Ubin
Thick-billed spiderhunter (Arachnothera crassirostris) - VR/R - BTNR, CCNR
Little spiderhunter (Arachnothera longirostra) - U/RB - BTNR, CCNR, P. Ubin