You might be interested in my xclorsel a Tk GUI to available X11 colors.

Not changed since 2007-05-08, so maybe apt for modernisation.

In relation: I have dis_colors to show the current setting for you ANSI-compliant terminal (xterm) related to colors. That makes it easier to choose the combination to select for your messages. This tool could do with an extension to show what to use from Term::ANSIColor, though that would make the display rather wide.

Note w.r.t. rgb_palette: I use light background and usually discard every tool that uses a black/dark background by default and has no easy/intuitive option to switch to light background. This excludes currently many IDE's as they default to dark background and their option(s) to switch to light background are not implemented well as obviously their developers do not use it. My own tools all default to something "nice" (YMMV) on a light background but when useful, a simple option or rc setting is supported for those that prefer a dark background.

It took me a bit to remember, but in the xterm source distributions come with color-tests in the vttests subfolder that show something similar to your rgb_palette.

EDIT: ExtUtils::MakeMaker is CORE and also comes with a prompt function, so you don't require an extra non-core dependancy.