How to get Undertale Coloured Sprites for free on Linux

You all know the video game Undertale. You are a little kid named Frisk that fell into the underground world of Monsters.

[SPOILERS AHEAD]
Long ago, monsters and humans lived together in peace. Then war broke out between the two groups. However, since the power of nearly all monster souls equates to just one human soul, none of the humans were killed by the monsters, while many of the monsters were killed. When the war ended, the humans brought out seven of their best magicians and created a magical barrier separating the humans from the monsters. Now the monsters are stuck in the Underground. Seven humans have fallen before you, and that is exactly the number required to break the barrier. However, the first human, Chara, has their soul inside Asgore's son Asriel. Asgore is the king of the monsters, and right now, he has six souls. Now he is trying to get your soul so that he can break the barrier and monsters can be free. (Only one human and one monster soul is required to cross the barrier and get human souls from a graveyard. But crossing it is too dangerous because humans can attack.) The goal of Undertale is to persuade Asgore to use all of the monster's souls and the six existing souls to break the barrier instead of killing you and using your soul. (Because remember, the power of nearly all monster souls equates to one human soul.)
[SPOILERS FINISHED]

Oops, I might have spoiled the game for you, but that's what you get if you don't follow the spoiler warning. 

Okay, so if you have ever played Undertale or watched videos about it, you will know that the overworld is in full colour, while the in-game battle sprites are black-and-white. This can be boring for some, so for those bored by that, I have the perfect solution for you. 

Just click this link, download the file, unzip it and read the README file. Or if you want to read it without downloading, here it is: 

By following the instructions below, you will be able to use Undertale coloured sprites on Linux. THIS WILL ONLY WORK ON VERSION 1.001. You can check your game version in the start menu.
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NOTE: THESE INSTRUCTIONS HAVE ONLY BEEN TESTED ON THE GOG VERSION
THEY MIGHT BE DIFFERENT FOR THE STEAM VERSION

1. Go to '~/GOG Games/Undertale/game/assets/'
2. Rename game.unx to game.unx.bak, so that you have a back-up of the original game in case things get wrong
3. Copy or move game.unx from this folder to '~/GOG Games/Undertale/game/assets/'
4. Boot up the game and you should have coloured sprites! :)
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Known issues: The X key will not work properly. However, you can use SHIFT for the same function and it will work fine.
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Brought to you by Mahaer Mahmud.

Note: this is not an article on how to install Undertale for free. You need to look somewhere else, sorry.
EDIT: The C key will also not work for opening your inventory. But you can use the Control (ctrl) key for that. And the Enter key will do the same functions as Z (even though the Z key will also work).