Not as much as the shape of the car but different finishes do affect drag.
Weight wouldn't effect speed but it would effect how quick it would get up to speed.
No, only the speed they are shot at.
when do you unlock paint
Are you kidding me? Of course! Red is the fastest!
The car's mass should have no effect on that speed.
They could: it depends on where they are placed.
Yes it will affect the speed of the toy car. Example: A buggy is easier to push without any groceries in it.
Aerodynamics, horse power, gear ratio,
effect of force
Metallic paint gives extra shining and rich effect to the car. Where as nonmetallic or solid paint, if polished will give glazing effect.
Unfortunately, the paint job will not affect the speed of the car.
Don't know how long ago you asked this, but I will answer anyway :) Check out this map: http://www.cheats.it/images/cheats/nfsumaps.jpg The red dots are Graphic Shops, you can paint your car there.