Replacing color - Only on one Layer

Hi,
I was wondering if it is possible to replace a color on one Layer without effecting the other layers? 

I was drawing pixel grass on 14 tiles that correspond to one another and wanted to vary the shadow, so I duplicated the grass layer. As it seemed easier to delete the shadows, I replaced the shadow-color with the background-color. Now all of my previous shadows on the other layers are gone as well.

WARNING when replacing color it will effect all LAYERS.

Cheers
rikki.o