Interesting conumdrum!
Of course the top colour of any paint (or decal) is dependant on what's underneath.
When you lay the decals down on the darker colour, it will darken the decal colour, which is why you have circular white decals to go under the roundels on the sheet above.
You may get some milage by applying the fuselage decal over your painted stripe, or remasking it, & overpainting with a lighter shade of sky?
The way I would approach this, is above, or...
PHOTOCOPY the decal sheet, tape it to your desk, lightly apply FRISK film, Semi-transparent & don't burnish the film, & cut your stencil.
Apply your stencil & then burnish the edges. Some people will spray a light gloss coat to seal the edges, any leakage being clear gloss & easier to clean up, then spray white, then Sky. lightly run a sharp scalpel round the edges to prevent peeling or ragged edges.