I used a ShareX's Screen Color Picker to find out the color of the maroon stripe you posted:
800000 got copied to clipboard.
Now I create a new project in Blender, go to Shading, and set this color for the cube:
the red component is $0.216$ which in 0-255 scale is $0.216 \times 255 = 55.08$, quite far from 128. But this is actually what you want to happen - Blender converts sRGB color space (BTW, it assumes you measured your color in sRGB color space which is not necessarily correct assumption) to linear color space, because while sRGB is perception-oriented, linear is physical-oriented, and Blender tries its best to be physically accurate. This accuracy is related to things like conservation of energy, however, the color you input is supposed to be the same when you output (render) your image.
...So why isn't it the same when you use a color picker on the cube?
The red is neither 128 nor ~55! The reason is the output is converted from linear color space to yet another, Filmic. To change it, go to Render Properties > Color Management and change View Transform to Standard
Now the color picker shows the same value. You can see it twitching and sometimes being off by 1 - this is due to dithering:
Why does Blender output noise even when rendering only flat colors?