My first Birkin was black with gh and I love, love, love it. But, it is summer now and it does seem strange to carry a large black bag (even Hermes) during a hot summer day. So now I've got gold one! So, if two are not an option, you may want to choose a color that you'll feel comfortable carrying all year. Blue (any H blue) would be divine.
I just got my first birkin : blue jean togo birkin 35 phw. And it's perfect time for summer color. But, when summer will end I don't think I'm gonna wear it. Try to think to buy black or taupe. So confused. I love taupe but black looks awesome!
Of your choices, I vote for Blue Brighton if you can find it. It's light enough to wear in the summer but dark enough to go with winter colors like black, grey, browns, etc. And a non-shocking or trendy color that will look good through the decades. But depending on your skin tone and wardrobe, I agree with others who suggest Etoupe (or Etain or Gris T);also consider a purple like Cassis or Raisin (again, if you can find it preowned) or one of the khaki greens like vert olive (personally my choice for a one and only Birkin color) or vert veronese. Good luck!
They are all beautiful choices! Black w GHW was my first Birkin and I find it transitions easily from day to night (mine in a 30cm). You will never tire of this classic. Some TPFers find that you can lighten the tone of it for summer by pairing with a brigthly colored twilly. You can always invest in a less expensive bag for summer like a GP or Picotin. Hermes does gorgeous reds so that is a great option too! Keep in mind, the "choice" may be decided for you based on what your boutique has available. Can't wait to see your reveal!! Happy shopping and best of luck!