*READ* "Pearl" Rarity is a Glitch *READ*

 The code has been studied- it's a glitch. 

Quoted from Gearbox forums:

I just did some perusing of my own on the matter.

Here's what I found, MULTIPLE places in the game files:

"WeaponPartCost0_Cheap"
"WeaponPartCost1_Common"
"WeaponPartCost2_Uncommon"
"WeaponPartCost3_Uncommoner"
"WeaponPartCost4_Rare"
"WeaponPartCost5_VeryRare"
"WeaponPartCost6_Legendary"

Also, found:

"Weight_Awesome_0_VeryCommon"
"Weight_Awesome_1_Common"
"Weight_Awesome_2_Uncommon"
"Weight_Awesome_3_Uncommoner"
"Weight_Awesome_4_Rare"
"Weight_Awesome_5_VeryRare"
"Weight_Awesome_6_Legendary"

Now, what does this mean? There are 7 distinct levels of rarity of items according to the game. Colors aren't mentioned here, but I assume they are:

-- White
-- Green
-- Blue
-- Purple
-- Yellow
-- Orange
-- Dark Orange

Also,

The rarity value is given an addition of certain variables, such as prefix, title, body, grip, barrel, magazine, stock, sight, acessory, material and so on.
Each one of those have different possible values, for most "generic" items it ranges from 1 to 5 for all of those and the higher the values for each variable more rarity points an item get.

You cant see how much rarity each part gives, the only way is testing each part on an identical weapon and compare the rarity level score on the debugger.

The white "pearl" items have a rarity level of above 100, that is due to mainly one variable, the accessory "thanatos" for example, that one alone gives a lot more rarity points than any other variable. And the game doesnt know how to handle over 100 rarity, therefore it treats it as a white item.

<snip>

 One word about the COLOR system.

Each preffix and title have an inate rarity value and tech value, independent of item quality/level and each part assembled on the item can have an effect on tech level and rarity level. Those can only be seen if you have debug mode activated, wich is a shame. 

101+ - White; bugged rarity values
67 to 100 - Dark Orange
63 - Orange
51 to 60 - Yellowi
25 - 50 - Never seen any on the debugger, maybe for class modes/shields and grenades
16 to 24 - Purple
3 to 15 - Blue
3 - Green
1 to 3 - White

The game assigns 'rarity points' on a scale from 3 to 100, depending on what parts make up any particular gun. Guns that make it over 100 are the product of a flaw in the Gear Designer.

So, for example, let's look at a hypothetical gun that only has 3 parts-

Level 1 Stock (5 points) + Level 5 Barrel (50 points) + Level 3 Scope (25 points) = 80 points = Dark Orange rarity.

So, that nice Thanatos Repeater you have has a particular suffix that has a few more points than it should, causing the rarity to go past orange and loop back around to white.

tl;dr

"Pearl" guns are a glitch in the game's Gear Designer.