How to clean grout over grout?
I seriously doubt that you will remove the dye from the grout using any method. Your best bet is to pick up a grout saw at lowes or home depot for about seven dollars that has a diamond blade on the end. You will be able to use the pointed end to file down the small amount where the gray overlapped( if that's what happened). IF it was too wet and these colors bled, then file down into the grout where it bled, mix your grout a little tacky instead of loose and touch up the edge.