I have added several plugins and got them all to work, all except button color.
I just added the color button folder to the editor plugin folder and then added config.extraPlugins = 'colorbutton'; to the config.js file. Pretty simple, but it does not work.
Have you cleared your browser
Have you cleared your browser cache?
Btw, using CKBuilder is a much better way to add plugins than solving issues manually.
Documentation Manager, CKSource
See CKEditor 5 docs, CKEditor 4 docs, CKEditor 3 docs, CKFinder 3 docs, CKFinder 2 docs for help.
Visit the new CKEditor SDK for samples showcasing editor features to try out and download!