sub calc_color {
  my ($i, $ncolors) = @_;
    $ncolors = 1 if $ncolors == 0; # No division /0.

  my $a = 0.95;     # tune these for the starting and ending concentrations of R,G,B
  my $b = 0.5;
  my $rgb = [ [$a,$b,$b], [$b,$a,$b], [$b,$b,$a], [$a,$a,$b], [$a,$b,$a], [$b,$a,$a] ];
  my $rgbmax = 125;   # tune these two for the outmost ranges of colour depth
  my $rgbmin = 240;

  my $n = $i % @$rgb;
  my $m = $rgbmin + ($rgbmax - $rgbmin) * ($ncolors - $i) / $ncolors;

  my @c = map { $rgb->[$n][$_] * $m } 0 .. 2;
  my $g = $c[0] * .3 + $c[1] * .59 + $c[2] * .11;
  my $f = $g > 127 ? "#000000" : "#ffffff";
  my $h = sprintf "#%02x%02x%02x", @c;

  ($f, $h);
}