Hi kankon,

Check this example. Now please take its reference and correct your code.

Database

For this example I have used of Northwind database that you can download using the link given below.

Download Northwind Database

HTML

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" OnRowDataBound="OnRowDataBound">
    <Columns>
        <asp:BoundField DataField="CustomerID" HeaderText="Id"></asp:BoundField>
        <asp:BoundField DataField="ContactName" HeaderText="Name"></asp:BoundField>
        <asp:BoundField DataField="Country" HeaderText="Country"></asp:BoundField>
    </Columns>
</asp:GridView>

Namespaces

C#

using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;

VB.Net

Imports System.Configuration
Imports System.Data
Imports System.Data.SqlClient
Imports System.Drawing

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        string query = "SELECT CustomerID,ContactName,Country FROM Customers ORDER BY Country";
        SqlCommand cmd = new SqlCommand(query);
        using (SqlConnection con = new SqlConnection(conString))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.Connection = con;
                sda.SelectCommand = cmd;
                using (DataTable dt = new DataTable())
                {
                    sda.Fill(dt);
                    GridView1.DataSource = dt;
                    GridView1.DataBind();
                }
            }
        }
    }
}

string prevCountry = "";
Random rnd = new Random();
Color color;
protected void OnRowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        if (prevCountry == "" || prevCountry != e.Row.Cells[2].Text.Trim())
        {
            color = Color.FromArgb(rnd.Next(0, 256), rnd.Next(0, 256), 0);
            prevCountry = e.Row.Cells[2].Text.Trim();
        }
        else
        {
            prevCountry = e.Row.Cells[2].Text.Trim();
        }

        e.Row.BackColor = color;
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim conString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
        Dim query As String = "SELECT CustomerID,ContactName,Country FROM Customers ORDER BY Country"
        Dim cmd As SqlCommand = New SqlCommand(query)
        Using con As SqlConnection = New SqlConnection(conString)
            Using sda As SqlDataAdapter = New SqlDataAdapter()
                cmd.Connection = con
                sda.SelectCommand = cmd
                Using dt As DataTable = New DataTable()
                    sda.Fill(dt)
                    GridView1.DataSource = dt
                    GridView1.DataBind()
                End Using
            End Using
        End Using
    End If
End Sub

Private prevCountry As String = ""
Private rnd As Random = New Random()
Private color As Color
Protected Sub OnRowDataBound(ByVal sender As Object, ByVal e As GridViewRowEventArgs)
    If e.Row.RowType = DataControlRowType.DataRow Then
        If prevCountry = "" OrElse prevCountry <> e.Row.Cells(2).Text.Trim() Then
            color = Color.FromArgb(rnd.Next(0, 256), rnd.Next(0, 256), 0)
            prevCountry = e.Row.Cells(2).Text.Trim()
        Else
            prevCountry = e.Row.Cells(2).Text.Trim()
        End If

        e.Row.BackColor = color
    End If
End Sub

Screenshot