Hey,

Actually just posting a solution which I hope will save someone some time.

I was looking to add the colour picker option to the RTE data type as well as some Header format options. Having tried out adding stylesheets and seeing some strange results in the Formats dropdown and rendering I decided to go with a combination from a couple of other posts.

Enable Colour Picker - https://our.umbraco.com/forum/umbraco-8/98024-how-to-add-color-and-font-size-featured-to-richtext-editor-umbraco8 (thanks to Saif Obeidat)

Set Custom Colours - https://our.umbraco.com/forum/using-umbraco-and-getting-started/107764-enabling-a-limited-font-color-picker-in-rich-text-editor (thanks to Mizzle's comment)

Add 'Headers' to Formats dropdown - https://our.umbraco.com/forum/extending-umbraco-and-using-the-api/87266-some-formats-in-tinymce-drop-down-are-disabled (thanks to Nicholas Westby).

There may be a better way of doing these but the combination from the answers on the above posts helped me out and does the job.

Thanks, Stuart