[FIX] website_sale: add color value on attributes

xml template was using color name, because color index was empty.
It works in English, but once translated (in french e.g.) we have background-color:noir

Fix #12251

addons/website_sale/data/demo.xml

@@ -808,6 +808,12 @@ Weight: 1.1 ounces</field>
         <record id="product.product_product_47_product_template" model="product.template">
             <field name="public_categ_ids" eval="[(6,0,[ref('product.Switch')])]"/>
         </record>
+        <record id="product.product_attribute_value_3" model="product.attribute.value">
+            <field name="color">#FFFFFF</field>
+        </record>
+        <record id="product.product_attribute_value_4" model="product.attribute.value">
+            <field name="color">#000000</field>
+        </record>
 
     </data>
 </openerp>