I often mix up black and brown and I think it can work very well. I would stick to all brown including footwear and just wear your black birkin with pride. If you can find something with black AND brown in it, as an accessory, then this might set it off. I have just bought an Hermes scarf with browns, gold AND black in it and I'm hoping it will bridge the gap between my opposite problem; black clothes and a brown (gold) bag. As your Birkin has GH, this scarf would look fantastic with brown clothes/black Birkin. It's 'sur en tapis volant' in colour no 3. I have the plisse silk scarf but I have also seen it as the large silk - it is gorgeous and a very unusual colourway for Hermes and even more gorgeous is the large cashmere and silk version in the same colourway. I will attach a pic of my Kelly - it has the plisse attached although it doesn't do the colours justice but it will give you some idea: