Taste the Rainbow!

For those who aren't already familiar with this fight when checking on this achievement:

- Yor'sahj periodically summons blood oozes to assist him, IIRC it's after 75 seconds in
- three (four on heroic) oozes appear at the edge of the area and creep towards him
- he absorbs them, then for 60 seconds, bad things happen based on the color of each ooze absorbed
- then he does it again
- only one can be killed each time, the other 2(3) will become immune, so normally you pick the most dangerous one to destroy -- for this achievement, you let him absorb these four specific color pairings

At 100, classes who can solo raids with ease can just stand there and wait while he summons and absorbs oozes for a while. Track the achievement so you can mentally check off which pairs have appeared, and when you've seen all four, finish him off.

[e]At 110, you're gonna have to hit him once and stop, since it's so easy to nuke him in less than a minute even on 25H.

A rainbow with Black in it?

.... Must be made by gnomes.

The Taste the Rainbow! achievement had be completed incrementally across multiple attempts. It is not necessary to get all the color combinations in a single attempt. In fact, many raids may find it easiest to just wait/focus on using necessary survival cooldowns to deal with a particular color combination in a kill one week, and then focus on another combination the following week, until all the requirements for the achievement have been met. Conversely, you can do more than one set of color combinations in a single kill, or even a single wave of adds (e.g., allowing the boss to get black/yellow/blue at the same time would get you the first two on the list below at once).

The impact of the pairings are as follows, generally ranked from easiest to most difficult:
[npc=55867] (Black) + [npc=55864] (Yellow) - Will result in two waves of [npc=56265] adds while healing through ongoing AoE from [spell=109549]. This is actually the easiest of the combinations.
[npc=55867] (Black) + [npc=55866] (Blue) - Will result in a wave of [npc=56265] adds, along with a [npc=56231]. Focus down the [npc=56231] when it spawns to get mana back for the raid, then finish the adds. As long as dps switches quickly to kill the [npc=56231], this combination too is rather easy.
[npc=55865] (Red) + [npc=55862] (Green) - Will result on the raid getting [spell=108356] that requires it to collapse, while simultaneously causing room wide AoE from [spell=108350] that splashes to nearby players. Generally, the raid will need to spread out at least 5 yards from each other to minimize splashes from [spell=108350], but positioned in a full circle around the boss to minimize the distance from the boss (and reduce the [spell=108356] damage). Plan to use personal survival and raid cooldowns to survive. Tank 4pc set bonuses may be especially helpful here.
[npc=55863] (Purple) + [npc=55864] (Yellow) - Will result in ongoing AoE from [spell=109549], while simultaneously dealing with the fact that the raid is afflicted by [spell=105171], causing multiple heals to result in damage at 5 stacks. Spread out to minimize Deep Corruption damage (although that will reduce AoE healing effectiveness too), and try to minimize heals until the debuff wears off for a particular player. Damage reduction cooldowns may be a helpful way to reduce damage without causing corruption stacks.

Notably, the last two options may be best to do by getting the boss down under 5%, waiting for the appropriate combination, and then killing the boss as soon as he absorbs those colors, ending the fight before you have to deal with a sustained duration of the unfavorable color combination.

It is important to note that to trigger credit for Taste the Rainbow!, the boss must absorb the slimes to get the appropriate color buffs; just killing him once the globules have spawned is not sufficient.