I don't know if you've considered it or not but if your application is based on coloring various groupings of items you should probably use the <optgroup> tag coupled with a class for further referencing. For example:
<select>
<optgroup label="Numbers" class="green">
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
</optgroup>
<optgroup label="Letters" class="blue">
<option value="a">A</option>
<option value="b">B</option>
<option value="c">C</option>
</optgroup>
</select>
and then in the head of your document write the css like this:
<style type="text/css">
.green option{
background-color:#0F0;
}
.blue option{
background-color:#00F;
}
</style>