Hi vinny, I'm not a t5 user anymore, so take my advise with a grain of salt. I've gone over to the dark side and use leds now. But I think t5 lighting is perfectly good for growing and showing off corals. In some ways better than leds and in some other ways, not as good. It's all a matter of personal preference.
However, I can try to help with the issue you asked about. Corals never look as good under white light, which is reflected off the corals pigments, versus how they look under blue light, which some coral pigments absorb, then do a chemical reaction and then fluoresce their own light.
You can add a lot of blue to the white in the tank and the overall color will get much more of a cool white or a high Kelvin temp like 16,000K or 20,000K. But the light fluoresced by the coral using that blue light energy will never compete with the white light reflected colors. The white just seems to wash away the much more vivid fluoresced colors.
Having leds I can adjust the blue and white power levels (in fact my fixture allows individual control of all 6 colors). If I run the fixture at 100% blue and 0% white the colors are crazy fluorescent and vivid. The tank looks like a 1960 hippi poster under a black light. By just adding 10% white to the mix and the corals look incredibly more pastel and faded. IMHO it's just the nature of the beast.
