sub calcColor ($$$)
{
  my ($min, $max, $value) = @_;        # min = the minimal (possible) value
                                       # max = the maximal (possible) value
                                       # value = the value you want a color for
  my $color;                           # be strict
  my $all = ($value < 0 ? $min : $max);# calculate with minimal or maximal value
  $all || return 'FFFF00';             # avoid divisions by zero
  $color = sprintf('%.2X', 255 - int($value/$all*255)); #calculate and make hex
  return ($value < 0                   # return
    ? 'FF'.$color.'00'                 # either a red
    : $color.'FF00'                    # or a green
  );
}