FWIW, this constructs a 216 MB file of random bytes in memory; then indexes it is 6-byte chunks to find 28 million "colors" in 37 million pixels in 56 seconds (and I'm guessing your machine is a lot faster than mine):

#! perl -slw
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => '_1138218', CLEAN_AFTER_BUILD =>0;
typedef unsigned __int64 U64;
typedef unsigned   int   U32;
typedef unsigned   char  U8;

#define TAG printf( "%s[%u]\n", __FILE__, __LINE__ )

HV* countColors( SV *img ) {
    STRLEN l;
    U8 *p = SvPVx( img, l );
    HV* counts = newHV();
    U32 i;

    for( i = 0; i < l; i += 6 ) {
        SV **val;
        if( val = hv_fetch( counts, p+i, 6, 0 ) ) {
            ++SvUVX( *val );
        }
        else {
            hv_store( counts, p+i, 6, newSVuv( 1 ), 0 );
        }
    }
    return counts;
}

END_C
use List::Util qw[ shuffle sum ];
my $data = '';

my @bytes = map chr, 0 .. 255;

{
    local $/;
    open O, '>', \$data;
    seek O, 226492416, 0;
    print O chr(26 );
    seek O, 0, 0;
    print O join '', ( shuffle( @bytes ) ) x 4 for 1 .. 221184;
    close O;
}

print length $data;

my $start = time;
my $href = countColors( $data );
printf "Took %f seconds\n", time() -$start;

printf "Found %u colors in %u pixels\n", scalar( keys %{ $href } ), su
+m( values %{ $href } );

__END__
C:\test>1138218-2
226713600
Took 56.000000 seconds
Found 28532475 colors in 37785600 pixels
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