Is the brownish substance sometimes observed in the gluteal and anal regions, accompanied by itching and moisture, sweat or an impure substance that nullifies ablution, and what is its legal ruling?

The summary of the fatwa is that if you are not certain about the purity or impurity of something, and its impurity is more probable, you are not obliged to wash it, nor is it ruled impure unless you are certain of that. It is not considered a nullifier of ablution unless you are certain that it exited from the private parts. However, the more cautious approach, and that which absolves one of responsibility, is to wash it to avoid disagreement.

However, if the color is accompanied by something that indicates it is feces—such as an odor—it is ruled impure, and one must purify oneself from it, and it is considered a nullifier of ablution. If it exits from you frequently and it is difficult to guard against it, then it falls under the ruling of سلس (incontinence).