I'm going to guess your hydro plants are a brighter green then a soil plant ? Maybe ?
This should be posted in the hydro folder. I'v never grown hydro and I dont visit the hydro folder of this forum cause I know nothing of it and cant give any usefull information at all. But since I'm bored, I'd have to say, if your asking about the color of green they are, maybe because hydro plants are suposed to grow faster, you have alot more new growth then a soil plant would. And it would give it a brighter "New" green all over ?