hey sumith,

i am a spray painter by trade... if your first question about maintaining your paint job is meaning in an accident or in years to come, then acrylic is a quick and easy job to do repairs and touch ups (small areas) but also easy to paint full panels and most people can do it at home.
note: Acrylic will fade in time say once every 3-5 years and will need buffing.

But if your looking into the metallic or pearl situation i would recommend useing the 2-part (2-pack) system. The clear gives the paint a longer life time (10 -15 year) if done correctly.

choosing either matellic or pearl comes down to you and money, pearls do cost a bit more but the different veriations are almost endless (eg. blue car with purple pearl, or blue and white pearl, etc,etc) were metallic is more the base colour in your case blue and with small metal flaks which brightns the colour up about and give it a sparkly look in the sun.

as for your second question, just plain black is fine! just make sure on your plastic items you sand and clean and use a plastic adhesion promotor before spraying the colour otherwise in time it will flake off and show the under colour, for the metal items just prepair them like it was a panel on the car or if new metal items you can also buy painted adhesion promotor, but read instructions before using them.!