alternate color for grid view when gird view having row span

Question

Hello i have grid view in that for one row and 3 columns. first two is as it is and 3rd column having column-span based on result set

ex
ID      category            sub category
1                sales              jan
                                       Feb
                                        march

2             production       Jan
                                       Feb

3                  release         jan



so i am using below code for alternative color but sub category wise alternative color comming .
i need alternative color base on ID and category not sub category.

Can you provide solution for this.

 

AlternatingRowStyle-CssClass="alt"

.Grid .alt       
{                     
background:#DEEDF5;       
}