Gaylen Gerber tests the principles of artistic collusion, collaboration, and commingling by occasionally inviting other artists to make their work on top of his own. There is a self-effacing quality to Gerber’s own painting, too: For many years he has been making monochromatic gray paintings, at first with the same, nearly invisible still life barely rendered on it. More recently, in a gesture of generosity, he’s taken to fittting the entire wall of a gallery with his signature slate gray as a support/ backdrop on which other artists may hang their work. This focus on the generic and unassuming