Plant turning white (temporarily)

This is a strange one, for me but I'm just starting so maybe it's not.

I added my first fish to my tank on Friday evening. 12 neon tetra. On Saturday evening I noticed that the heads at the top of my Limnophila Heterophylla had turned white just before I switched the lights off and went to bed. When I got up in the morning they were green again so maybe I was asleep and was just dreaming?

Yesterday I added 3 Gold Laser Cory to the tank and just 5 minutes ago I noticed that the heads were turning white again. Half an hour before they definitely weren't as I was taking pics of the fish and would have noticed. They are a bright white and I don't doubt that when I go back down in half an hour they will be an even brighter white.

Does anyone know what's going on? Should I be worried? It isn't my lighting is it?

It looks like light reflecting off the leaves. Are you sure they are turning white? Perhaps turn the light off and see if they are green. If they are green straight after you turn the light off, then it's light reflecting off the lant leaves.