You can set the BackColor
too in using the data binding syntax
<asp:TextBox ID="txt_ReasonCode"
onchange="disableNextStatusButtons()"
runat="server"
Text='<%# Bind("ReasonCode") %>'
Enabled='<%# (Roles.IsUserInRole("İhracat Uzmanı") && Session["Status"].ToString()=="3") %>'
BackColor='<%# (Roles.IsUserInRole("İhracat Uzmanı") && Session["Status"].ToString()=="3") ? System.Drawing.Color.Red: System.Drawing.Color.Green %>'
Width="40px">
</asp:TextBox>
A little ugly, but will work just fine.