Return an alist of colors supported by FRAME's terminal.
FRAME defaults to the selected frame.
Each element of the returned alist is of the form:
(NAME INDEX R G B)
where NAME is the name of the color, a string;
INDEX is the index of this color to be sent to the terminal driver
when the color should be displayed; it is typically a small integer;
R, G, and B are the intensities of, accordingly, red, green, and blue
components of the color, represented as numbers between 0 and 65535.
The file `etc/rgb.txt' in the Emacs distribution lists the standard
RGB values of the X colors. If RGB is nil, this color will not be
considered by `tty-color-translate' as an approximation to another
color.