A few of you have noted in comments that there are rarity levels past “pearlescent”, this is both correct and incorrect.
The valid game rarity levels are defined as such: (a screenshot I took from Borderlands UnrealEd several weeks ago, later modified by someone else to show actual colors)
Weapons normally range from 1 to 100 rarity, when guns fall past 100 rarity (due to either a bug or the developers didn’t think weapons could go past it), they become white due to not being in a valid rarity. “Pearlescent” weapons are in fact a bug due to this. Past this we see special rarity levels used for different types of items, such as medpacks, etc. If a weapon manages to reach those rarity levels, it will have those colors.