Hi Chatelaine, I seem to remember it as a darkish shade of cerulean blue, maybe with a touch of prussian or purple blue. Why don't you go to an art shop or the art dept. of your local diy store, they should have colour charts from Talens or another make watercolours and go to diy store with that?
What a lovely thing to think about...garden sheds...
I'm smelling the freshly-cut grass at the moment. Nothing beats the first ventures into gardening each year does it?
Enjoy your day!