I have a VBA code in excel to change colors of bar graph but its not working for category series.
ActiveChart.SeriesCollection(1).Interior.Color = RGB(0, 15
Your problem isn't with the VBA, to have different colored bars they must be on different series.
Leave a gap in the base colored series and add the values you want colored on a second series and color that. Your data would look something like this:
Series | Month 1 | Month 2 | Month 3 | Month 4 | Month 5 | Month 6 | Month 7
1 10 12 15 14 10
2 17
3 18