Tetrachromancer

Took over 100 hunts to gather these. They can drop from the farmable white satchels as well.

Edit: Forgot to mention that they are account wide. Looted the last one on another toon and got the ach.

Droprate for these items seems to be much higher in your first weekly hunt reward.
Since this achievement is account wide, you can use any of your lvl 60+ alts to get these items without having to spam 100 hunts.

This is also a great way to get Plainswalker Bearer mount.

Just to help those who are wondering if it is feasible to get the achievement for the meta achievement in time before the reward is taken away as it has been in past expansions - it categorically is NOT statistically probable you will get this achievement, or the meta. Blizzard needs to remove this, or change the drop rates and we all need to provide this feedback to blizzard.

Statistically, in the same period of working a full time job to get this achievement you are more likely to:
Win a small lottery prize
experience a significant earthquake
witness a meteor shower.

TLDR: Doing 15 Grand hunts everyday for 5 months, will net you this achievement statistically.

of course my maths may be wrong but you are going to need to put in some serious work and you immediately need to be farming grand hunts, upwards of 30-50 grand hunts a day. to get this in time, some many more if you aren't lucky.

Based on everyone's comments and anecdotal accounts and my personal experience of doing maybe 15-20 a day for the past 2 weeks (as this is one of the last achi's i need for the whole expansion) my maths below adds up to what you can expect.

I personally would be at maybe 300-400 hunts and the drop rate was just stupidly thought out, and noone thought to check if this meta achievement would be achieved, even by those playing consistently from day 1. It is the last achievement I need. Based on probability, everyone should just give up on the meta achievement.

The Weekly purple bag, has a drop rate of ~1.8% for each colour - 1 in 450 attempts
The Blue bag, has a drop rate of ~1.2% for each colour 1 in 750 attempts
The White bag, has a drop rate of ~0.4% for each colour. 1 in 1 in 2250 attempts.

Since there are 9 colours just doing the weekly purple Sachel it will take you approximately 450 weeks.. or 8.6 years.

If you want this achievement for the meta - DO NOT JUST DO THE WEEKLY HUNT

Considering a reasonable human maximum of doing 15 grand hunt attempts per day, we can calculate the number of weeks it would take to collect all colours:
Given:
• Once a week, you get 1 attempt at a 1.8% drop chance.
• Once a week, you get 1 attempt at a 1.4% drop chance.
• 104 times a week, you get a 0.4% chance per attempt.
Calculation:
1. For the 1.8% drop chance: Expected attempts per week = 1 * 1.8% = 0.018 chance per colour
2. For the 1.4% drop chance: Expected attempts per week = 1 * 1.4% = 0.014 chance per colour
3. For the 0.4% drop chance: Expected attempts per week = 104 * 0.4% = 0.416 chance per colour

Total expected chances per week = (0.018 + 0.014 + 0.416) attempts = 0.448 chances per colour

To collect all 9 items, With an expected 0.448 chance to a colour per week, Number of weeks = 9 items / (0.448 chances/week) ≈ 20.09 weeks

depending on when this meta achievement will no longer be available, you may just need to give up even if you have done every other achieve for the expansion.

Therefore, it would take approximately 21 weeks to collect all 9 items under the given conditions, doing 15 grand hunts a day and the purple and blue sachel – approximately 5 months of constant farming grand hunts in your play time and nothing else!

Its a good idea to pass this feedback to blizzard, the vast majority of players, even if this is their last achievement, and even if you have consistently done 2-3 grand hunters each week from launch (which is a healthy amount given other things to do tin the game) will likely not get the meta. Seriously disappointing.