<NewColor>

The first  is based on the actual definition of the <Color> parameter.   It uses this to reference the current colour.

The other form is <Color> / <Color> which sets forth a specific area and fills it with two colours in a checkerboard pattern, or used for text output for the foreground and background colors respectively.

Example :

In a 16 colour version of a vehicle a combination of dark red and black can be used to simulate very dark red: #800000/#000000