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If it was me, and I was going to be painting it again, I'd strip it back to bare and start fresh.

could be the grey primer making the yellow look darker, could be the paint itself.

When we paint r/c car bodies we use white as a last coat to brighten them up, or black to make them dark.

It's...

What primer did he / you use?

and what colour was it?

don't forget to take into account that the rest of the paint has been exposed to the sun for years and the bumper is fresh paint.

Plus, from most of the yellow Zeds I've seen, they never matched...