I don't know if this has already been reported. At least, I could not find an entry on this particular issue. If this is already known, I apologize for the double entry.
My observation is about color settings in the Paragraph Style, Character Style and Group Style dialogs. There, the dialogs with the sliders do not accept any keyboard input. It is only possible to make settings with the sliders.
I can reproduce this behavior
* in Affinity Publisher 1.9.2
* inAffinity Designer 1.9.2
* with the RGB color sliders
* with the CMYK color sliders
* in each case in the color settings for paragraph styles, character styles, group styles
* on my MacMini 2020 on macOS 11 Big Sur
* on my very old MacBook Pro 2008 on macOS 10.11 El Capitan.
For clarity, I've included a short screen video that also shows my keyboard input.
Color_Sliders.m4v