I am stuck on a blending problem that appears to have only started once I started blending without any color. I am painting a grey suit and using shades to capture the lighting realistically. For some reason, when I paint with a dark grey over a light grey, with say 20% opacity, with enough strokes, the color I am painting will match the color in the color picker. With the reverse situation (light to dark), the paint tool never quite blends to the color in the color picker, it is always a shade or two off. No matter how many times I stroke the area, it will not become the color I have chosen. It has me dumbfounded and is crippling my ability to make light and shadow and show depth.
I have tried googling and messing with every possible option, deselecting all, triple checking what layer I am in, but I cannot seem to find anyone else with this problem...
I openned GIMP 2.8 (stable version) and the development version of GIMP and tried a procedure like the one you tell about: Indeed, when working with 8 bit color, the wayGIMP is structured internally will prevent the gray values to converge toa precise shade of gray in same cases. When GIMP applies 20% of the difference between a value 129 to a pixel valued 127, that "20%" is a "0.4" darkening, which is rounded down to zero.
This certainly won't be dealt with on current GIMP stable, since it is fundamental to the way 8 bit color works, ansd given that GIMP unstable - the 2.9 version that will eventually be out as GIMP 2.10 can be set to use higher color precision so that this behavior does not happen. (With floating point pixel values, you just get as close as you want from your shade of gray).
I'd suggest you either find a compiled "nightly" version of GIMP 2.9 for your system, or try some other way of painting: maybe using a more spread shade of gray with values varying over a broader range, and after you are done, compressing the results to the desired range with the Levels or Curves tool.
Anyway, this is offtopic here - if you have further doubts on painting, please take the question to graphicdesign.stackexachange.com