I'd like to change my '73 back to it's original color, Zambezi Green.

But I've been thinking about painting the rear trunk Black,
instead of Z Green.

Has anyone else done something similar, or can think of a reason
why this would Not be a good idea?

I did it. Well, my 914 has orginally been white. PO had painted it red. It was painted pretty well. All trunks and so on. But, I didnt like it. I wanted something darker.

I painted both trunks, engine compartment and bottom of the car gloss black. From the outside it is painted dark metallic pearl blue. It is original Porsche color from 1992, I think...

So, IMHO it is ok to paint the trunk black. But, the other color should match with the black. In my 914 the blue is very dark and I think those two color matches together ok. I am not sure if your Z Green would do that.

Inside the boot (trunk) ? or the boot lid?. Some of the 914-6 GTs had black on the inside...

This or this?

I painted my rear boot in gloss black, but I agree with Timo, if your colour suits black (like a dark blue) then it can look good. I don't like it now its done so I'm going to repaint it yellow to match.