I went o my bro's convo last yr, and I think the area just outside the convo hall and the long staircases directly opp of tha convo hall and the flowers near it are quite nice. I dunno wats the convo's location called. Its the one with the golden mummy statue(s).
Pick a place with a strong colour theme that goes well with the blue gowns.
A largely green background with some yellow/magenta/red would be nice. Quite easy to find in NUS.
A building with strong colour theme (esp yellow) might work too, esp if the building is quite new looking... i've not been back in NUS for a long while, so have no idea what they've been building these past few years.
If outdoors, try morning light (b4 8am) or wait for the soft orange sunset light. Have to recce beforehand and work fast - the light fades very fast.
If you're picking an indoor+outdoor scenery in one picture, wait for the 5 minute slot where the light level is equal both indoors and outside. Otherwise, outdoor portion will be blown, or indoor portion too dark.