Looking at the settings in your screenshots, the culprit of the color dilution would seem to be the brushtip’s Gaussian mask type (Gaussian is a blurring method).
As for single-pixel smudging, I’m not sure why it’s seemingly unpredictable. I think I would just use color picker and a regular 1px brush for that, personally. (For multipixel pixelart smudging, I might recommend rxb) Pixelclaws from Rakurri Brush Set V2 - Free Krita Brushes 😁 .)