asp:button hover color change

Given the following HTM:

 <asp:Button ID="ImgSend" Text="Send Email" BorderStyle="Groove" BorderWidth="1" BackColor="Navy"
                                                            Height="28" Width="100" Font-Bold="true" Font-Size="14px" ForeColor="White" runat="server"
                                                            OnClick="ImgSend_Click" OnInit="ImgSend_Init" />
and codebehind:

    protected void ImgSend_Init(object sender, EventArgs e)
    {                                                                                                          
        ImgSend.Attributes.Add("onmouseover", "this.style.backgroundColor='silver'");
        ImgSend.Attributes.Add("onmouseover", "this.style.color='navy'");
        ImgSend.Attributes.Add("onmouseover", "this.style.cursor='hand'");
    }

I test the code and see only cursor "hand" but the colors don't change. Please advise.
Thanks.