This has been debated before (not that we can't again) but there are actually a lot of different ways that loot are color coded in different games.

What most of them have in common is this;

Grey < White < Green < Blue < Purple < Yellow < Orange

Diablo 3's system is a bit different, as they look like this;

• Junk - grey
• Normal - white
• Magic - blue
• Rare - yellow
• Legendary - orange
• Set - green

Now some games the developers want to differentiate themselves from the masses, but it's hard for us to adapt to that because let's face it, we have been used to this system since early WoW (*) and Diablo 2 (**)!

I've proposed it before and thus I will add my cents to this - keep it as recognizable as possible. This will make it easier for new players to enter into the game, as they will know which rarity the loot they find has just by the color coding from the get go:

White - Normal

Green - Master Crafted (former blue)

Blue - Rare (former green)

Purple - Artifact

Orange - Godlike

And OP, it's Master Crafted because the weapon WAS crafted ;)

From my own perspective the green colour looks more visually impressive than the blue (just the colour stands out more to me) and therefore it makes sense that it's more important. 

Given the current system I think Red would be best suited for the upper most tier. Similar to vermin-tide. 

My only gripe with the current naming convention is that master crafted should swap with rare in terms of power levels so from weakest to strongest we would have common, rare, mastercrafted then artificer I am hoping there will also be relic items with unique abilities/perks to top out the power levels (more or less Inquisitor Martyr's version of a diablo legendary)

also green being right under blue isn't a wow thing.  It is that way in wow, but it's not like they invented the color order, wow and diablo were just the games with the largest user base that used it and thus it caught on.