Some new achievements i thought of
1. fart of all farts: add a fart command that makes you shoot a green see through gas from your butt
An interesting concept. However I notice two flaws in your post:
first, you say you thought of achievements. But, you only list one. This demonstrates a lack of clear instruction in what a "plural" is and when you should, and should not use it. Unfortunately, I'm not a teacher and certainly have no experience helping the mentally handicapped, so I am in no position to try to rectify this at the current time.
Second, while the achievement you do mention has some negligible advantages, I think it is outweighed by it's immense idiocy. Of particular note are several things- why would the player be able to fart, but have no need to defecate or urinate? This would certainly consume the thoughts of minecraft players everywhere. Additionally, the various gasses excreted in what can be called a "fart" are colourless; as such my first instinct upon hearing the words "green fart" are "hazmat team" because chances are it's a cloud of chlorine or something. Or, also likely, it could be a fresh minty scent. Of course, neither chlorine, nor fresh mint are known to reside in the anus of your typical human, so this sort of puts your idea in a spot.
An additional question might be whether this is single player or Multiplayer, and why you would need a command in Single-Player. that is just bad design. It should have a configurable key-binding, changable in options. You need to think these things through. Of course if you had thought it through you would know it was idiotic, but hindsight is 20-20.