This command initiates a dialog that allows the user to specify two colors: the High Color and the Low Color.  The program uses these two colors to create a smooth range of colors that will stretch from Transition Bottom at the low range to the Transition Top at the high range (see Levels…).   Clicking on the box representing either the  High Color or the Low Color will bring up the standard ColorPicker allowing the user to select the desired color.

When running on 8-bit color (or any clut type display), Fractal! will set the color palette to the most appropriate one for the 32-bit  image  just rendered.

Note first of all that both Transition Top and Transition Bottom are parameters that can be set from the Levels… command in the Appearance menu.