Dear Anis, please see Ayrat Dimiev's papers on the chemistry of GO for a proper answer to your question. Mn species have nothing to do with these color changes.
The color of the GO gradually changes from yellow to dark brown with repeated washing of the GO with water. This color change is due to the significant chemical transformation in the course of purification that result in increased conjugation of the pi-system and thus darkening of the GO. The dark brown color of the water washed GO is an indication that the sample absorbs in a wide region of the visible spectrum. An increase in the size of the conjugated pi-domain, the increase in chromophores in conjugation with these domains and conjugation of previously isolated pi-domains can all be reasons for the brown color of the GO.
First of all, at the time of synthesis the acidic mixture contains lots of MnSO4 impurity. The color of this specie is pale pink ( especially, in case of tetrahydrate ). So, initially due to this GO looks more bright and faded white. After the continuous rinsing this impurity get rid of along with some labile oxygen functional groups. Moreover, at the time of washing dissolved oxygen of the system (either in acidic water or between in the layers of GO) also deprive/reduces. Due to this some how crystallographic lattice spacing (001) also decreases as compare to original exfoliated system after adding of water.... simply means reduction process happen which finally led the color change as per pi conjugation and chromophore theory.
Dear Anis, please see Ayrat Dimiev's papers on the chemistry of GO for a proper answer to your question. Mn species have nothing to do with these color changes.