Protection Label's watermarks are editable?

Hi, 

We are rolling out information protection labels using Purview. 

I noticed the limited number of colours for watermarking content within the label setting. This was intriguing as I don't understand why a colour hex value couldn't be supported... Especially as the yellow is extremely hard to read, if I was to change it to readable orange that would be better.  

I hope am wrong, watermarks are just editable textbox withing the header and footer of word doc? I can change text and colour, save the document and the label's watermark are tampered with just like that?

I was hoping that the label's watermarking wouldn't be editable in a word doc and embedded. I understand the label can still provide technical controls, but tamper proof water marks would be useful.