CRL Wool Daubers are an industry standard for the application of pinchweld primers and glass preps prior to installation of automotive replacement glass. Colored rags or towels can contaminate the primers if used. The primer can leach out the dyes or dissolve nylon reinforcement fibers in the cloth towels.
Always use clean daubers whenever you apply primers or cleaners. Never put a contaminated dauber back into the primer bottle as this contaminates the primer.