The Italian tricolor was first established during the Napoleonic Wars by French republics in northern Italy , who styled it after the French tricolor.
In 1848 the design was adopted by the house of Savoy, which went on to lead the Italian unification. The present flag was adopted in 1946, when Italy became a republic and the royal arms were removed.
I don’t know of any authoritative and indisputable explanation regarding the meaning of the colors of the Italian flag . The most credible theory suggests that the colors came from the colors of the uniforms of the Civic militia of Milan which were predominantly green with some white. Some red parts were added in 1796 when the Militia became the National Guard. In October of that year, the Lombard Legion was constituted, which adopted a uniform with the same color combination.
Orders issued October 9stated that “each Cohort will have its National tricolor Lombard standard, differentiated by number, and ornate with the emblems of Liberty”.
It is said that Napoleon himself described the colors in his message to the “Direttorio” (directorate) dated October 11, 1796 in reference to the constitution of the Lombard Legion
“… the national colors to adopt are the green, the white and the red”
The original standard had the colors displayed vertically with the green at the hoist, representing the very first (known) model of the Italian “Tricolore”, although, at the time, it was only known as a military flag.
The first true (displayed) national flag in which the colors were applied horizontally is the "Cispadane Republic" flag. This flag was adopted on January 7, 1797 and had the red at the top, the white in the middle, charged with the coat of arms, and the green at the bottom.
On July 17, 1797 the Cispadane and the "Transpadane Republics" were united into the "Cisalpine Republic", which adopted the Tricolore, known today as the official Italian flag.
It is for that reason that the official version generally claims that the Italian flag is modeled after the French Tricolore.
In 1802 the Cisalpine Republic became the Italian Republic and on August 20, 1802 a new flag design was adopted. The new design was modeled after the Napoleonic military flags: on a red field a white lozenge under a green rectangle. On March 1805 the republic became a kingdom at which time a golden Napoleonic eagle was added on the flag’s green field. The Tricolore, charged with the Savoy shield in the center, became the national flag of the Kingdom of Sardinia in 1848 and of the Kingdom of Italy in 1861. The State flag and the war ensign also included the Savoy crown.
Finally, on June 19, 1946, the plain Tricolore became the official flag of the Italian Republic. In order to avoid confusion with the Mexican flag at sea, on November 9, 1947, two different coat of arms were added by decree , one to the civil and one to the war ensigns.
The official colors were finally established on January 17th 2003 and are expressed in Pantone colors : Green (brilliant grass) 18-5642TC, White (milk) 11-4201TC, Red (tomato)18-1661TC.