Products/Ingredients that DARKEN hair

I suspect your lemon shampoo says "clarifying" somewhere on it?  If so, it is not meant to be used every wash, but only occasionaly to remove buildup.  They make shampoos that are like paint in the bottle (not the see through kind) that are color depositing shampoos.  I've seem them on occasion in the dye isle of the store, but usually you would have to get them online or at a beauty supply shop.  The brown would likely darken your hair a good bit, especially if its been colored in the past and is porous.