Change the color of grid view for 2 rows of set

Hi,

I want to change the Background color of GridView according to different serial no. eg in below grid there r 2 rows for 1 serial no. I want to add differant color for new serial no.
e.g for s/n 002 color=gray and for s/n 004 color white. I am tryind but i am able to change for alternating row not acoording to serial no.


__________________
s/n | Dir| Bus No|
____|____|_______|
002 | 1 | 77345 |
----|----|-------|
| 4 | 78954 |
----|----|-------|
004 | 5 | 7897 |
----|----|-------|
| 1 | 67895 |
----|----|-------|
007 | 3 | 67843 |
----|----|-------|
| 2 | 54321 |
----|----|-------|

Thanks

Pallavi