The shade in the corners when with a brush looks darker than in the rest of the wall where the paint has been applied with a roller.

POSSIBLE CAUSES
The paint you hear with the brush has not been diluted more than that applied with the roller.
There is no good compatibility of the pigments with the color.

THE SOLUTION

Applying paint with a brush generally leaves more paint on the surface than the roller. This provides more coverage so that the shade looks darker.
Dilute the paint to be applied to the “cut” with a brush, more (eg 10% instead of 5-7% with water). Apply as thin a strip of paint as possible to the cuts.