Light has to both render color in the coral and then also illuminate them to your liking. 6500k makes corals color up really nicely. 6500k also makes them all look a bit yellow. RB diodes make coral look really good, but do not really color them up very well. This is why people compromise when they choose how to light... I go with a 14k MH in my display which both color and renders very well without supplements. I use 6500k and sometimes 10k in my frag tank since I only care about lots of output. Some people use to use 6500k balanced them with a ton of Actinics.
The best example of this are corals at a frag show... they are grown under whiter, more daylight conditions to really render a lot of great color and contrast, then they are illuminated at the show with RB diodes only. People see them under those diodes and think that this is how they look, when most of the time the color shifts when the more daylight source is kept away... they fade over time.
Another example is to look at shallow water SPS photos in the wild. These have tremendous color but look kinda bland under daylight only. If you were to put these under a 20k bulb, then they would shine like crazy... purple and pinks would radiate, some of the yellowish green ones would probably be blue, etc.
The best example of this are corals at a frag show... they are grown under whiter, more daylight conditions to really render a lot of great color and contrast, then they are illuminated at the show with RB diodes only. People see them under those diodes and think that this is how they look, when most of the time the color shifts when the more daylight source is kept away... they fade over time.
Another example is to look at shallow water SPS photos in the wild. These have tremendous color but look kinda bland under daylight only. If you were to put these under a 20k bulb, then they would shine like crazy... purple and pinks would radiate, some of the yellowish green ones would probably be blue, etc.