There are 3 methods to add a new Color Mapping:

Keycut

The keyboard shortcut Ctrl+M is the default keystroke sequence that will add a new color mapping. This keystroke sequence can be remapped by selecting "Define Shortcuts" from the Options menu.

Tools Menu

Select "Color Mapping" from the "Tools" menu. This will display a popup menu in which you should select "Add Mapping".

Document Window

Right click within the document view and select "Add Color Mapping".

Color Mapping Attributes

Hex Sequence	Defines the byte sequence in hex displayed in a special foreground and background color. The maximum byte sequence is 64 characters; any data entered beyond 32 bytes is ignored.
Foreground Color	Selects the foreground color used to display the specified hex sequence. This foreground color is used for both the hex and text portions of the Hex Workshop display.
Background Color	Selects the background color used to display for the specified hex sequence in both the hex and text portions of the Hex Workshop display. "Use default background color" must be unchecked to specify a background color.
Use default background color	Defines whether a user-supplied background color or the default background color is used.