It is the H2O2 which changes the solution coloer from brown to yellowish. I think you need to add only a few drops of H2O2, I dont know how much you are adding.

thank you Mr. Sahito, I add 1.875 ml H2O2 and125ml water to my solution which already contains ( 0.75 gr graphite 2.25 gr KMnO4 17.5 ml sulfuric acid and 33 ml water)...

Thank you Mr. Tiwari

Thank you so much Mr.Tiwari and Mr.Halder for your kind help.

I am pretty certain it is not due to the oxidation time, however it could be due to the amount/freshness of kmno4 added. 

This color change isn't a major issue, i faced the same issue repeatedly(greenish yellow color instead of bright yellow), upon characterization i found that the GO synthesized was perfect so I would suggest you characterize it and see.

Thank you dear Aisha...I will do that... I have synthesized another sample a few days ago and I have left it for 4 days before adding H2O2 ...and surprisingly I saw that even before H2O2 addition the stirred solution was green again!...That's weird because at first it was dark brown (  + a little dark green) but after 4 days it got totally green which wasn't that much dark...

You are adding a high amount of KMnO4, reduce that to around 8 grams. This will solve it

Thanks Mr.Sahito but I add 2.25 grams that's a wight ratio of :  KMnO4/Graphite = 3

Seyede Farzana.. Did you find out any thing? I tried to do the hummers method and at H2O2 addition stage my solution turned from brown to yellow to green.

Here is video of my reaction

https://youtu.be/NthPpaVCgd4

Hashim Shah

Farzane Besharat

I can tell you from the experience that in your cases the reason is nothing but the temperature of your mixture when you pour H2O2 into it. You must increase T of the mixture before adding H2O2 via adding water or putting the jar of mixture into a high temperature bath, mostly 90 or 98 is used as far as I have researched, other temperatures above 45 degrees of centigrade are OK as well. Moreover, be careful about adding water step, because it is the second step of oxidizing graphene sheets. For more information you can look at link below.

Article The Hidden Second Oxidation Step of Hummers Method

I am also synthesizing graphene oxide. My mixture turns yellow (orange-yellow but seem stale) immediately after I added H2O2. I washed with 30% HCl, then 4 times of Water already. The pH is at 7.0 but the mixture is now a slightly dark orange gel. I'm so confused what's going on. I can't get my GO to the dark color.