You can assign the controls in the td with an ID, Loop through the items in the repeater control and then use FindControl("<id>") to find the respective control and assign the background color property or change the css.
If the above didn't work, as an alternative
You can create a Label control inside the td and bind the text to it. Now try finding the Label control and change its color.
<td>
<asp:Label ID="lbl1" runat="server" Text= <%#Container.DataItem("title")%>> </asp:Label>
</td>
foreach (RepeaterItem item2 in Repeater1.Items)
{
if (Convert.ToInt32(dr["Quantity"].ToString()) > 1)
{
Label lbl= (Label)item2.FindControl("lbl1");
lbl.Attributes.Add("style", "background-color:Green;");
}
}