my %colour_min, %colour_max;
@colour_min{ 'red', 'green', 'blue' } = $widget->rgb('black');
@colour_max{ 'red', 'green', 'blue' } = $widget->rgb('white');

# ...

my %triplet;
@triplet{ 'red', 'green', 'blue' } = $widget->rgb($user_entry);
my %slider; # 0..1 range [eg. 50% grey might be (.5,.5,.5) ]
for my $channel ('red', 'green', 'blue')
{
  $slider{$channel} = ( $triplet{$channel} - $colour_min{$channel} ) / ( $colour_max{$channel} / $colour_min{$channel} ) );
}