If you can't get the correct paint, then, using the image of the Humbrol tin of 'Sky' as a guide, lighten the 65 with white until it is a very pale blue, then add a spot of yellow and a spot of green (any green) until you get an approximation to 'Sky'. It's much easier with an actual sample to work from though, and easier still if you can get the correct paint or equivalent to Humbrol numbers. As you presumably haven't built the kit yet, then the painting stage is some way off, so might be worthwhile shopping around and obtaining the correct shade.