Since user_notification_template=""
is a parameter, and there is no form field for it, it isn't possible to modify this setting once the form has loaded. I recommend submitting this as a Feature Request under the specific add-on category. http://support.solspace.com/support/discussions
This may be far-fetched, but you could use javascript to reload the form with a new user_notification_template value after selecting one of your options. The value if the user_notification_template would be passed in a segment in the URL.
For example, if you select "Red", the form reloads with a URL link this (this is an example): http://mysite.com/contact/redTemplate
Then, in your {exp:freeform:form}
(or {exp:freeform:composer}
) parameters, use the segment in your user_notification_template=""
parameter, i.e.:
user_notification_template="{segment_3}"
Of course make sure to have a redTemplate
notification template set up in the CP.
Be careful of parse order! Turn on Template Debugging if necessary