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If it's a steel or alloy frame I would have it sand or plastic media blasted. Sand blasting should only be used on a fairly solid steel or alloy frame though.
Often the blaster will spray an etch primer on the frame. Make sure it's suitable(ie sticks well in the long term) for alloy. It should be fine for steel.
That'll give a good base for the undercoat.
It depends if you're going to do it yourself or getting someone else to paint it.
If it was me and it was to be a training/commuter bike I would be inclined to get a HPC finish - a high performance coating that is thin but is a very hard finish and will last for years.
Otherwise a two pot finish would be best. Baked enamel is softer than two pot so don't go there.
If you're going to have someone else spray the topcoat you could go to a car painter to get it done and ask what colours they have in stock. If you use what they already have it will be(should be!) cheaper as you wouldn't have to buy a new can of paint - usually sold only in 1 litre containers which is way too much for a frame. 1 litre of topcoat will do about 1/3 of a small car!
If a two pot topcoat is used and applied well it won't need polishing etc to look good - it should be shiny straight off the gun.
You could use a basecoat(not shiny when sprayed on so the clearcoat can stick to it, it's just the colourcoat) first and a clearcoat on the top. If you want to add decals/transfers do it on the basecoat and the clear coat goes over them too.
I hate two colours on bikes(with rare exceptions). A frame is too thin, small to support two colours visually.
Bright colours are fun and safe too. I get more waves riding my bright yellow(hence bigbananabike user name) than I do on my dark grey one.
There are sites etc on the net re spraypainting - you'll find good information.
Happy reading, Paul