Lookin to mix up a batch of spitfire paint

Okay it's winter, the snow's deep and the freakin hobby shop is 43 miles away. I realize any modeler worth their superglue would hop on a snowmobile an get there but I think I'd rather mix my own; uses up all that left over green and brown.
Here's the deal- I'm gonna paint a 1/48 spitfire mk 1 and I have lots of different green's such as french khaki, usmc, russian armor, euro 1 dark and a few others. I've a good selection of brown's like brown, light tan, field drab etc. all these are model master enamel's and I will airbrush them, the trick is the mix, how much white to add to euro 1 dark green to get the green for the spitfire. Same with the brown (dark earth). The real problem (for me anyway) is what color is "sky" ? It appears to be a "cream" or light buff type of white, is there a little blue or green too? No, I'm not going to buy color chips, the PE, bulged tires, masks and exhausts are enough expenditure for this project.. I realize this is a paint question and should be in that forum but it is airplane specific. Anybody out there been down this road before? Thanks guys....
G.L.