The MatID and ObjectID channels output multicolored material / object silhouettes. This is useful for easier selections in a 2D image editor when you want to edit only specific materials in the rendered image.
You can change the material ID color that will be assigned to a particular material from the "Material Properties" row in the Material Editor in Studio/MXED. Please see the documentation for your plugin to find out where this parameter is located.
You can change the Object ID color that will be assigned to a particular object from the Object Parameters panel in Maxwell Studio - "Appearance" section.