So you need to condition the sort test result on quantity and colour.

use strict;
use warnings;

my %cat;

while (<DATA>) {
    next if ! /^(\d+)\s+(\d+)$/;
    $cat{$1} = $2;
}

my @order = sort {
    $cat{$a} && $cat{$b}
        ? $a <=> $b
        : $cat{$a} <=> $cat{$b} || $a <=> $b
        } keys %cat;

printf "%-3d %-3s\n", $_, $cat{$_} for @order;


__DATA__
70 3
72 10
74 8
76 11
77 6
90 12
91 8 
92 10
93 9
78 0
75 0
73 0
71 0
[download]

Prints:

71  0  
73  0  
75  0  
78  0  
70  3  
72  10 
74  8  
76  11 
77  6  
90  12 
92  10 
93  9
[download]

The ternary operator ?: selects compare by colour if neither count is 0, or $cat{$a} <=> $cat{$b} || $a <=> $b which first compares by count then (if both counts are the same) compares by colour.

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