Right!
Sounds like its going to be a bloody nightmare :lol:
Your best option will either be to get the primer spectroed or match it up with colour chips. If you can find a bodyshop or supplier with chromatic chips this will speed the process up as they are arranged in shade order rather than grouped by manufacturer.
You'll want to sand back the bare metal and apply an etch primer, then apply your colour matched topcoat (which will be the colour of the OG primer, wait for that to go off and flat that to achieve the look you desire. I'm still not sure it'll look right though as it'll be on top and feathering the edges rather than showing through from underneath, and then apply the laquer to the entire panel. I have a feeling you'll need to do some experimentation with matting agents in the laquer to achieve the correct gloss level to match the rest of the bus.
It would be far easier just to paint it body colour, blend the basecoat across the panel and laquer the whole lot edge to edge.