I've typically painted cars/bikes in bright colours with acceptable results.
This time I want it to look as close to professional as I can get it (without renting a booth).
My most recent project was a flat black and turned out disastrous.
I was told that the darker to colour the more the imperfections show. Can I conclude from that that white would be the colour that is easiest to make look good?
In the same vein, would it be harder to detect imperfections in a shiny colour over a flat/matte colour?

 

Depending on what you are doing a booth is a big part of a good paint finish. There is nothing worse than doing all the right prep work and having bugs or pieces of fluff getting into the paint.

Next after that I would say a clean air supply is next in importance. If you don't have a fancy compressor setup make sure you use a filter near the gun that will catch water droplets which will cause fish eye in your paint.

Then for paint make sure whichever paint you choose you have the proper breathing apparatus. Some paints you are safe wearing the proper cartridge respirator but with some paints you need to have an outside, clean source of air pumped to your mask.

I think flat finishes are some of the most difficult because I don't know of a touch up or buffing technique you can use to fix boo boos. There may be some touch up tricks for flat paint but I don't know them. So, flat has to be perfect right from the beginning. A gloss paint or a base and clearcoat you have the option of buffing out runs or imperfections.

Yellow is one of my favorites but in addition to being expensive seems to require another coat to get it to cover properly so there is no see through. Surprisingly traditional metallics aren't too bad if you follow the mixing instructions and keep the paint in the gun stirred or moving. I've found white and black to be about the same though I've only done black as a single step and only in places where it didn't have to look 100%.

 

Bottom line, paint is not a cover up for imperfect body work.

If the prep work is krappy the paint will be also regardless of what is used.

 

Flat black takes a little more skill to spray than most gloss paints. It's imperative that it be sprayed evenly as unevenly applied paint can appear to have a different sheen/color.

If I remember correctly the old lacquer finishes were kind of flat until buffed out. They are also fairly forgiving but they don't dry to as hard a film which means they don't wear as well. Probably the main reason they are no longer used except for show cars.