Indeed.

You may, nevertheless, wish to fix the fact that when $v >= $vmax both the original and the updated code give (r=  0 g=  0 b=  0) cf the expected (r=255 g=255 b=255).

As to performance, I ran a little benchmark over 1,000,000 random values -200..+200:

                  s/iter colorRamp1785_u colorRamp1785_2 colorRamp1785_3
  colorRamp1785_u   7.26              --            -19%            -65%
  colorRamp1785_2   5.85             24%              --            -57%
  colorRamp1785_3   2.54            186%            130%              --

[Colour reproduction and colour perception being what they are, I imagine there's a cunning way of mapping a linear scale to something which appears linear. I just wish I knew what that was :-(]

Update: FWIW the fastest I can make a direct look-up is the code below, which benchmarks:

                s/iter colorRamp1785_u colorRamp1785_1 colorRamp1785_2 colorRamp1785_d
colorRamp1785_u   7.23              --             -8%            -20%            -67%
colorRamp1785_1   6.62              9%              --            -13%            -64%
colorRamp1785_2   5.79             25%             14%              --            -59%
colorRamp1785_d   2.37            205%            179%            144%              --

BEGIN {
  my @map = (
    ( map rgb2n(      0,       0,      $_), (   0.. 255) ), # => (    
+ 0,      0, 0..255)
    ( map rgb2n(      0, $_- 255,     255), ( 256.. 510) ), # => (    
+ 0, 1..255,    255)
    ( map rgb2n(      0,     255,  765-$_), ( 511.. 765) ), # => (    
+ 0,    255, 254..0)
    ( map rgb2n($_- 765,     255,       0), ( 766..1020) ), # => (1..2
+55,    255,      0)
    ( map rgb2n(    255, 1275-$_,       0), (1021..1275) ), # => (   2
+55, 254..0,      0)
    ( map rgb2n(    255,       0, $_-1275), (1276..1530) ), # => (   2
+55,      0, 1..255)
    ( map rgb2n(    255, $_-1530,     255), (1531..1785) ), # => (   2
+55, 1..255,    255)
  ) ;

  sub colorRamp1785_d {
    my ($v, $vmin, $vmax) = @_;

    return $map[(1785 + 1) * ($v - $vmin) / ($vmax - $vmin)] 
                      if ($v >  $vmin) && ($v < $vmax) ;

    return $map[   0] if ($v <= $vmin) ;
    return $map[1785] ;
  } ;
} ;
[download]