Yes,I used H2SO4 , NaNO3, KMnO4 and Graphite and stirring them in an ice bath for an hour and one more hour in 35 C.But its color doesn't change much!

You have caution in proportion of acid and peroxide if possible look this paper in Supporting Information have description about synthesis and change color by steps. Also have another steps after 35ºC you have dilution acid with water and stirring by 2 hours. More details in

http://pubs.rsc.org/en/content/articlelanding/2013/RA/C3RA40412A

Its color is due to KMnO4. kMn2O7 is formed in low temperature in concentrated H2SO4 solution.

Based on my experience, following to the modified Hummers method which basicaaly has three stages of temperature, at this stage of temperature ( 35ºC), it didn't show much changes in terms of color, where it turns to greenish brown (but it still dark color though). The subsequent processes do change the color, i.e addition of water, hydrogen peroxide, and you even get a very nice brilliant yellow as described in the previous reports.

Santanu said correctly, the oxidation reaction is the slow process so you should add KMnO4 very slowly then only it will oxidize completely and you will get brown colour.

See this paper

http://pubs.rsc.org/en/content/articlelanding/2010/cs/b917103g in attach

Green color is appeared when KMnO4 is added to solution. so, the color can't be from graphite bi-sulfate , I think so.

see this

http://en.wikipedia.org/wiki/Manganese(VII)_oxide

Mn2O7 arises as a dark green oil by the addition of concentrated H2SO4 to KMnO4. The reaction initially produces permanganic acid, HMnO4 (structurally, HOMnO3), which is dehydrated by sulfuric acid to form its anhydride, Mn2O7.

2 KMnO4 +(cold) 2 H2SO4 → Mn2O7 + H2O + 2 KHSO4

Mn2O7 can react further with sulfuric acid to give the remarkable cation MnO3+

Mn2O7 + 2 H2SO4 → 2 [MnO3]+[HSO4]−+ H2O

Mn2O7 decomposes near room temperature, explosively so at > 55 °C. The explosion can be initiated by striking the sample or by its exposure to oxidizable organic compounds. The products are MnO2 and O2.[6] Ozone is also produced, giving a strong smell to the substance.

your observation is correct.. do dilution with minimum amount of water after after 2h stirring at room temperature. after dilution,solution turns to reddish brown colour. React with hydrogen peroxide after the temperature treatment for 30min at 60oc you will get brilliant yellow color. this is my practical experience

the color of aniline/Mgo changes from green to brown

can anyone help me in understanding the mechanism of modified hummer's method and how can i switch to green solvents (both for reduction and oxidation)