Areas inside the set are usually filled in black, but this is only a convention; you could color them in differently to make the fractal look more interesting. The only method available to make areas inside the set visible is to display the value of the latest orbit as the value of each pixel.
The tutorial on incoloring has more information and examples.
XaoS has many different ways to show that value. The cryptic names of the modes are mathematical formulae, where real means the real part of the latest orbit, and imag means the imaginary part. zmag uses the magnitude of the value. The Decomposition-like method uses the angle of the orbit. Also, truecolor incoloring modes are available, that display one value in each of the red, blue and green color planes (or, for some modes, in each of the hue, saturation and value planes).
In the scripting language, the incoloring mode is specified by one of the following integers:
Truecolor. To set exact parameters for truecolor coloring use the ''tcolor'' command.
Available as: command line option, command