BEGIN {
    my %map = (
         255 => sub{         0,             0,   $_[0] * 255 },
         510 => sub{         0,     $_[0]*255,           255 },
         765 => sub{         0,           255, (1-$_[0])*255 },
        1020 => sub{ $_[0]*255,           255,             0 },
        1275 => sub{       255, (1-$_[0])*255,             0 },
        1530 => sub{       255,             0,     $_[0]*255 },
        1785 => sub{       255,     $_[0]*255,           255 },
    );
    my @map = sort{ $::a <=> $::b } keys %map;
    sub colorRamp1785 {
        my( $v, $vmin, $vmax ) = @_;

        ## Peg $v to $vmax if it is greater than $vmax
        $v = $vmax if $v > $vmax;

        ## Or peg $v to $vmin if it is less tahn $vmin.
        $v = $vmin if $v < $vmin;

        ## Normalise $v relative to $vmax - $vmin
        $v = ( $v - $vmin ) / ( $vmax - $vmin );

        ## Scale it to the range 0 .. 1784
        $v *= 1785;

        ## And look up the appropriate rgb value
        ## And pack that into a 32-bit integer compatible with GD truecolor
        $v < $_ and return rgb2n( $map{ $_ }->( $v % 255 / 256 ) ) for @map;
    }
}

##</code><code>##

my $plotColor = colorRamp1785( 2.5, 0.5, 10 );