I agree with xdusty. If you have a high load of organics in the water, they could brown out a little. Also, lighting has a lot to do with coloring. And it also depends on the individual animal.
Many of my Acans are more colorful under higher lighting, but I have 4 or 5 Acan lords that are more colorful under lower lights. I have one morph that get's very pink and purple under a 175 watt halide, while the same morph under 400watt halides is more marroon/brown. It's the same water running through both tanks, but if you saw them both you would never believe they were the same coral.
I think a lot of the wild Acanthastrea lordhowensis are dull colors at first, but could end up coloring up with some TLC.