The really correct way to do this is to subtract the given value from the largest cofficient, then multiply the smaller cofficients by the quotient of its difference. This way, all coefficients are decreased proportionally, so that unlike with BrowserUK's method the hue never changes, only the luminance. (Actually, it's not 100% correct either, but very close.) 
sub darken_hexrgb {
    my ($hexrgb, $darken) = @_;
    my @rgb = map hex, unpack "A2"x3, $hexrgb;

    my $max = (sort {$a <=> $b} @rgb)[-1];
    return "000000" if $max <= $darken;

    my $fact = ($max - $darken) / $max;
    return sprintf "%02X"x3, map { int($_ * $fact) } @rgb
}

[download]

That's what it looks like:

darken_hexrgb("FF9944", 8) = F79441
darken_hexrgb("FF9944", 16) = EF8F3F
darken_hexrgb("FF9944", 32) = DF853B
darken_hexrgb("FF9944", 48) = CF7C37
darken_hexrgb("FF9944", 64) = BF7232
darken_hexrgb("FF9944", 80) = AF692E
darken_hexrgb("FF9944", 96) = 9F5F2A
darken_hexrgb("FF9944", 112) = 8F5526
darken_hexrgb("FF9944", 128) = 7F4C21
darken_hexrgb("FF9944", 160) = 5F3919
darken_hexrgb("FF9944", 176) = 4F2F15
darken_hexrgb("FF9944", 192) = 3F2510
darken_hexrgb("FF9944", 208) = 2F1C0C
darken_hexrgb("FF9944", 224) = 1F1208

(Layout shamelessly stolen from BrowserUK.)

Makeshifts last the longest.

In reply to Re: Hex color degrader by Aristotle
in thread Hex color degrader by Kage

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