What color to paint or spray first?

Certainly it usually works best to spray the lighter color first. In this case the lighter color is a gloss paint, which takes longer to be dry enough to take the second coat. I would wait at least a week before spraying the Olive Drab, and I would be careful with masking on the gloss light blue, since some tapes may stick very tightly to the smooth gloss surface, and might damage it or leave residue when removed. I'd use a low tack masking product, or even Parafilm M, and remove it as soon as the Olive Drab is dry.

First spray the flat colour. I know its the darker of the two, but gloss paints will go over flat paints easily. The same can not be said for flats going over glosses, they just don't adhere well that way.

If you do decide to put the gloss paint on first, make sure youv'e got a good solid primer coat under it and put at least two coats of the gloss paint on. Next, give the gloss a VERY light sanding until the shine is gone, make certain to pay special attention to the edges of the painted areas and sand them down so they're smooth and you can almost see the primer under them.

After that, wipe the sanding dust off with a damp cloth, let it air dry and then mask as you need to for the flat colour.

Only put down your clear sealant coats after all your other colours have been applied. this will keep your final gloss or flat finish consistent on all areas of the finished model.

I know, it sounds like a lot of work, but its more work to descibe it than actually doing it. It works and its worth the effort.

Happy modeling