As long as it is red:

The History of why Italian Autos are Red

At the beginning when engines would not reach 18000 rpm, and tires were just larger than that of a bicycle, the Federation assigned the official colors of the racing cars depending on the nationality of the car manufacturer.

Italy was assigned red.

It suited us, since everybody knows that red cars go faster. Either at Spa, Nurburgring or Monza the public could recognize at a glance its team automobile, like a sort of championship of the nations.
Blue was the Bugatti and the Talbot, green the Bentleys and Jaguars, red the Alfa and the Ferrari since Italy's color was red. Nowadays advertisement has changed everything. If the sponsor can afford it he or she can order the car any color  he/she choose, like at a dealership.
Of course as long as it is not a Ferrari, which is and will always be red.
Because Italy was assigned red.

Purché sia Rossa.

All'inizio della fiera, quando i motori non giravano a 18.000 e gli pneumatici erano poco più larghi dei tubolari da bicicletta, la Federazione assegnò i colori ufficiali delle vetture da competizione, secondo lo stato di appartenenza del costruttore.
L'Italia ebbe il Rosso.

by Luca Ciai