For each channel (e.g. diffuse, reflectance, etc) the user can assign a color, a bitmap texture or a procedural. Clicking on the palette icon to the right of each channel will display the list with the available options.
Color: Use the color selector to define the RGB values of the color. Color also has the Spectrum option that allows you to define RGB values in the visible spectrum.
Bitmap: You can use tonemapping (Gamma, Saturation, etc.), change the projection, and mix the bitmap with a color.
Random Color: Applies a random color to instances in the scene
Procedurals: Procedurals have their own customizable settngs.