In Definitions edit, and some other windows, there is a colored square.

Click the square. The Windows color picker will allow you to choose a custom color.

Basic colors: Click on a color square to select it.
Custom colors: To change the color, drag the +, drag the arrow, or enter color values below. Custom colors cannot be saved.

Click OK to select the color and close the window.

Troubleshooting

If the right portion of the Color window doesn't initially show, click Define Custom Colors.

If the color black will not select, choose a different color, click OK, then reopen the Color window and select black.