extensibility for enumeration item in simple type

Hi All,
How can I extend the enumeration item in simple type. 
e.g. If I have following definition:
<xsd:simpleType name="Color">
    <xsd:restriction>
        <xsd:enumeration value="red"/>
        <xsd:enumeration value="yellow"/>
        <xsd:enumeration value="blue"/>
    </xsd:restriction>
</xsd:simpleType>

Though I want the user to define extra colors, so can I write like this:
<xsd:simpleType name="Color">
    <xsd:union memberTypes="xsd:token">
    <xsd:simpleType>
        <xsd:restriction>
            <xsd:enumeration value="red"/>
            <xsd:enumeration value="yellow"/>
            <xsd:enumeration value="blue"/>
        </xsd:restriction>
    </xsd:simpleType>
    </xsd:union>
</xsd:simpleType>

Is there any other way to write this definition?

Thank you for your help.
Avin Patel