Change Items styles in a DropDown Combo

Hello, I want to change the style of some items of a drop down list web control from the server side with the followintg example code:

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liAux = new ListItem("TEST", "0");
liAux.Attributes.Add("bgcolor", "red");
Dropdownlist1.Items.Insert(0, liAux);
Dropdownlist1.Items[0].Text = Dropdownlist1.Items[0].Text +
                                                    liAux.Attributes.Count.ToString() +
                                                    liAux.Attributes["bgcolor"].ToString();
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the result HTML code is the following:
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<select name="Dropdownlist1" onchange="__doPostBack('Dropdownlist1','')" language="javascript" id="Dropdownlist1" class="cbGeoGreen11">
      <option value="0">TEST1red</option>

</select>
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where's the bgcolor attribute of the option item???? but as you can see the text of the item contains the attribute so the item have the attribute... so the HTML should look like this:

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<select name="Dropdownlist1" onchange="__doPostBack('Dropdownlist1','')" language="javascript" id="Dropdownlist1" class="cbGeoGreen11">
      <option value="0" bgcolor="red">TEST1red</option>

</select>
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I have tried with CssClass with the same result.

Could anyone help me with this trouble?

Thanks in advance.