Hi
The only way you will get a metallic paint job to look the same over the entire car is to bolt it all together
and paint it all at the same time, Go the whole way along the side of the car in one go , Don't paint rear 1/4 then door then front guard.
The main problem with the colour being different when you paint the car separately is the thickness of the paint. I hear you say but i put 3
coats on everything but even a few microns different depth of paint will make the metallic particles sit differently. Then there is the angle of the gun to panel,
It goes on and on. You must paint the car as a whole.
Paint all the openings and under the boot bonnet etc assemble the car then colour the outside, Then clear.
You should try matching paint these days with 3 layer pearls etc etc. Not fun
Regards
Paul