Primers come in various coulours, red-brick, black, light grey and white being the most common I think. What your final coat of paint will be will generally dictate what colour of primer you use. For a red Ferrari, or any light and glossy colours, you'd better use a white primer as it will help your final paint getting its brightness. On tanks, wargaming figures (Lors of the Rings and such), I often use black primer as it will go towards giving me a 'washed' look after application of the camouflage (black possibly 'showing through' in corners and other recessed areas. For many of my airliners, the white and light grey primers are used, often as final coats, only to be re-coated with a satin varnish (Klear/Future). On normal figures, I'll use white as it's the easiest one to cover with the flesh colours.
Automotive primers are fine, I use a UK brand called Halfords, but be aware that some may attack the plastic so always try on a scrap or sprue first!