I'm a new user of Krita, and new to digital art in general.

I've looked into the various uses of the blending modes, and I am keen to test them out.

I am having trouble with activating the color related blending modes both from the brush tool and the layer tool.

When I follow the list to the particular blending mode I am interested in (say HSY > Color), the option has a square 'speech bubble'-like icon next to, and I can neither select the option nor tick it as a favourite.

I can imagine I'm making a basic error, or missing a basic step, but I haven't been able to find any guidance online about this...?

Many thanks to y'all who can offer help!

You are probably creating the image with CMYK color model.
CMYK is a subtractive color model. The more color you add, the darker it gets.
Opposed to RGB color model wich is additive. The more color you add, the lighter it gets.
Some blending modes will not work with CMYK.