The green sinu bleaches out under heat stress(I lost a large colon from chiller failure 2 years back). If the nutrient is overly rich, the color comes in a little faded. I have not seen mine fade because the colony got big. Mine also seem to get greener if I give it more light.
Hope you get it to bright up. One coral similar to yours is called palau nepthea. I have both the palau and green sinu, nepthea is head and shoulders more neon green. You would like the palau more but it is difficult to find.
this is a great question. i just aquired a large neon green nepthea colony, which was very green at the lfs, and moved it into my tank (400w halide w/ actinics. 60g cube) and have not seen the color come back yet, its been in the tank for about 3 weeks now. so i am giving it time to regain the color. i am noticing a hint of green returning on the branches, but i am still waiting to see.