what's it mean when sps are showing color only were light hits the coral underside wh

[BrentH]

Not sure how to describe what I'm talking about I'll try and post pics I am having low p04 issues and my n03 is around 10 p04 0.00 it seems like it's effecting the corals were they don't get light or maybe to much light I'm not sure

 

[trueblackpercula]

This is normal for SPS to have lighter colors on the underside.