If you have an airbrush, use it.
If it were me, i'd mask around the areas to get the stripes that is, the total stripe area.
Then i'd a/b the lightest blue colour over the total area.
Get some masking tape and mask the area to be left light blue.
Then paint the total area the medium blue.
Mask the area to be left medium blue (so now you've got light blue painted and still masked, and medium blue painted and masked) then paint the dark blue.
Take off all your tape - should be nicely painted.
If you use a brush, this method might not work as well, as the area with the darkest blue would, by then, have about 4 coats of paint - base, light, medium, dark and hence you might lose a bit of detail underneath.
Hope this helps.