That web.config setting is no longer used it was a temporary solution until the config file was implemented.
You should copy the contents of the latest mojoTinyMCE.config into your CustomTinyMCE.config file and then modify it.
You will see I have added a new toolbar for the Newsletter and this toolbar uses the forecolor and backcolor toolbar items so you can copy some settings from it to the other toolbars where you want to use it.
Note that it requires not only the correct toolbar items but also the correct plugins for those toolbar items, look at the NewsletterToolbar settings as it has the correct ones.
Hope it helps,