A couple of things:
1) The attribute tag should look like this:
2) Make sure that your public MBeanInfo getMBeanInfo() method is providing an array of MBeanAttributeInfo objects that contain color as an attribute.
3) Make sure that your setAttribute(Attribute attribute) method can set the color attribute.
That should do it.
It makes no different how the mbean is implemented. The jboss-service.xml attribute setting simply gets translated into a setAttribute call through the mbean server. There is no value attribute on the attribute element so as Nicholas said the value needs to be passed as the text body of the attribute element.