it has a lot to do with the SPS. some of the big branching SPS's, like staghorns. you can get away with, but do not expect fast growth or bright colours. the lower the light the more zooax that the coral will need to produce food. zooax are brown, so that will be the predominant colour of the coral. i have seen acros grow under NO lights, not the prettiest things, but it can be done. of course this has a lot to do with the depth of the tank and the amount of PC lighting you are doing. if you completely fill a canopy over a 20 long with PC's than you will prolly get some good growth and colour, but try the same thing on a 110g tall, you will not be satisfied.
in other words, do what Casey said.
G~