British racing green, or BRG, is a color similar to Brunswick green, hunter green, forest green or moss green (RAL 6005). It takes its name from the green international motor racing colour of the United Kingdom. There is no exact hue for BRG – currently the term is used to denote a spectrum of deep, rich greens. "British racing green" in motorsport terms meant only the colour green in general – its application to a specific shade has developed outside the sport.
When Britain first competed in 1902, they had to choose a different color from the national flag colors of red, white and blue, because those had already been taken for the 1900 race by America, Germany and France respectively. When Selwyn Edge won the 1902 Gordon Bennett Cup race for England in his Napier it was decided that the 1903 race would be held in Ireland, at that time a part of the United Kingdom, as motor racing at the time was illegal in Great Britain. As a mark of respect for their Irish hosts, the English Napier cars were painted shamrock green. (More info Wikipedia).
