HEX code field greyed out after new MS forms update

Hiya, 

My MS forms updated to (what I believe is now) the latest version of Microsoft but since the new update, the HEX code field to personalise the background colour is greyed out and can't be selected. (screenshot attached)

When I view this on existing forms that I had already changed the colour on, it allows me to update it, but any new forms or forms that are still only set to the default colour, doesn't allow me to update it. Is this a bug or is there a new feature I need to turn on somewhere? Is anyone else having this issue?

This option is imperative for any forms I create to ensure I follow my companies brand guidelines. 

Thanks 

Sophie