Dear Shivaani Srinivasan thank you for posting this interesting technical question. DAD (3,3'-Diaminobenzidine) is known to form a brown, insoluble product when it gets oxidized. For more information about this please have a look at the follpwing useful link:
In DAB staining, DAB is oxidized by hydrogen peroxide in a reaction typically catalyzed by horseradish peroxidase (HRP). The oxidized DAB forms a brown precipitate, at the location of the HRP, which can be visualized using light microscopy.