SUMMARY
If a brush is using a tip that is less than 2 pixels in diameter, it becomes impossible to add color settings to the brush. which means, it's impossible to add a hue randomizer and such other settings to a default pixel brush.

to solve my issue, i had to make a texture myself that is 3 by 3 pixels, import it as a predefined brush tip and add a hue effect to get my desired brush, thanks to someone on krita's reddit helping me.

we would like this to be fixed as it would make the lifes of pixel artists and other artists a little easier.

best wishes.

SOFTWARE/OS VERSIONS
Windows: windows 10 pro, last version of krita which is 5.2.2