Thank you for the replies. Since no one seems to be able to replicate this issue I recorded the issue to give a better Idea of what's happening:

Video:
https://youtu.be/84LA570cJno

Seem still working for on v17, just tested on runbot.odoo.com

https://youtu.be/KC4NC8CZj7E

Hi,

Please try using !important along with your style. By using it provides important to your newly given style

example:color : #FFFFF !important;

Hope it helps

Using Odoo online when I customiza the backgroun color of a block by clicking in the colored circle it let me type the hex code to black and white. what versión are you trying on?