$workbook->set_custom_color(40, 255,  102,  0   ); # Orange
    $workbook->set_custom_color(40, 0xFF, 0x66, 0x00); # Same thing
    $workbook->set_custom_color(40, '#FF6600'       ); # Same thing

##</code><code>##

$workbook->set_custom_color($back_color, "#$sRGB");