
In order to provide an accurate visual representation of the color
chosen with fl_color_chooser, color 15 is being set to the rgb
values you pick. The button changes to the new color, as does any
other widget using color 15. The number 15 is an arbitrary one
chosen simply because that color is not being used elsewhere.

fl_color_cube is used to find the closest match in the 0-255
range to the RGB color picked with fl_color_chooser. It has
a somewhat complex formula, and I'll probably never use it
in a real-world situation. In fact I wouldn't even use RGB
colors as part of a FLTK interface, simply because the 256
provided colors are plenty. RGB/hex is of course useful in
finding colors that will be used for other purposes, such
as examining full-color images or passing color values to
other programs.
