On win32, a color is defined as a 24-bit number, bits 23 to 16 define B, bit 15 to 8 define G, and bit 7 to 0 define R. see this web page for details.

The formula you want is: (B * 256 + G) * 256 + R. You can wrap it in a sub.

use strict;
use warnings;

print rgb(255,255,255), "\n"; #this gives you 16777215
print sprintf("0x%x", rgb(255,255,255)), "\n";
print rgb(255,0,0), "\n";
print sprintf("0x%x", rgb(255,0,0)), "\n";

sub rgb {
    return ($_[2] * 256 + $_[1]) * 256 + $_[0];
}
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