It is important to know that matrix effects can only be used with Rects. It is not possible to drag this kind of effect onto a regular fixture selection. Read the "Rects mode" chapter to see how to use Rects.
Once the matrix effect has been selected and dragged to the timeline, a window is opened and you will have to select the type of effect (square, circle...).
Several options are available: number of colors, size and speed. It is possible to adjust these settings with the corresponding rotary buttons. Once the number of colors has been changed, you can select the colors from the color picker.