Every time Match! needs to assign a new color (e.g. to a reference pattern entry when it gets marked in the candidate list for the first time), it uses the colors and line styles that are defined on this page of the "Colors and line styles" dialog. (The "Colors and line styles" dialog can be opened using the corresponding button in the toolbar at the top of the main window.)
The colors and line styles are used from top to bottom; if a color is already used by some reference pattern, the color and line style from the next line below are used etc.
For all default settings, the current color is displayed in the corresponding box. On the right-hand side of each color box you will find a button "Modify...". By pressing on the button you can open a color selection box where you can select/define a new default color.
It is not only possible to define the default color but also the corresponding line style and width. The current line style is displayed to the right of the "Modify..." button; if you press on the small arrow on its right-hand side, you will get a list of available line styles from which you can also select the new line style.
To the right of the line style box, you can enter or adjust the corresponding line width (given in points). Either use the "up" and "down" array buttons to modify the line width, or enter a new value directly.