Single colored operations are performed by the Solid function. It's the first function that is selected after startup and used for most standard image editing procedures.

The pixels of the texture are replaced by the primary color (left).

Solid's flexibility comes from the Mask settings. It enables the function to draw lines, rectangles, circles and other shapes. Also flood fill and color replacement operations are done by selecting the corresponding mask mode.

Change the operation transparency with the Mixer. It defines how the function result (i.e. the color here) and the underlying texture are mixed. A second texture can be set as Opacity Map, which means that it works as additional mask. A paper structure can be loaded with the Variator Map settings.

The Mixing Mode is very important for the outcome of the function. Image 3 for example was done with the mode Add and  Gradient Ellipse as mask mode to create the glowing effect.