Very nice job, it's good to see this kit built up and looking so good when finished.
Paul (DaddyO) summarised the approach to "ghost" roundels very well. On my Bristol Scout build I sprayed the base CDL, then cut masks for the basic roundel shape which I sprayed in a light grey colour. I then cut separate masks for the blue and red coloured parts and sprayed a very light coat of coloured inks to give a hint of the red and blue colour showing through. This might be overkill in 1/72, you could just go with the single grey colour to represent the whole roundel, up to you.
The trick is to use blu-tak or similar to hold the masks slightly off the surface to get a nice feathered edge. Then do rib tape shadows (so they go over the ghosted roundel), I revesre masked these in 1/32 but using a market pen or pencil like Paul suggested is fine in 1/72. Finally, overspray in CDL to taste to make the whole effect look more transparent.