Shader: Fill Part of Texture?

Hello, I'm working on a 2D shader that will be used in a GUI. It involves an alpha texture masking the main texture. I need to fill parts of the mask texture based on a certain property. For example, imagine that the mask texture is separated into 4 quadrants. I would like to fill one or all of the quadrants with white (as to not mask that area) based on the properties _ShowQuadrant1 ... _ShowQuadrant4. How can we fill only part of the mask texture with a solid color using a shader? Thanks in advance.