Weapon rarities go in the order White -> Green ->Blue -> Purple -> Light Orange -> Orange -> Dark Orange -> Red -> Pearlescent.
Pearlescent, initially, was a minor glitch, but since the addition of 'The Secret Armory of General Knoxx' DLC, it has been a real rarity. Most of the weapons will be white at the early stages of the game, with the rarity being low. By the end of the second playthrough orange becomes the common rarity. The rarity is indicated by the colour of the name of the weapon in the invetory menu.
The more people you have in your Party, the higher the chance of higher rarity weapons and the lower the chance of lower rarity weapons, Alas, this is quite hard to prove.