I've also been trying to get this working in Kentico 8 for awhile without any success.
I think Brenden supplied the wrong plugin for CKEditor, as the UIColor is for altering the color of the editor box itself and not the HTML elements. I think the correct plugin should be ColorDialog. http://ckeditor.com/addon/colordialog
I've tried adding the files to the plugin folder and adding
config.extraPlugins = 'colordialog'; to my config.js file, but it still doesn't appear.
If I try adding the lines Juraj recommended, my CKEditor disappears completely.
Can anyone help?