In Monster Hunter World you unlock the option to change armor color / pigments during 6-star assignments. After unlocking 6-star assignments on the quest board you must play the next main quest to slay a Pukei-Pukei and then the color change becomes available. At the same time, you will gain access to high-rank armor at the smithy in Astera. When this happens you will also receive a short notice/tutorial about changing armor pigments.
This is relatively late into the game so beware you must make a lot of progress for this to work.
Once you have high-rank armor unlocked simply go into your room in Astera and interact with the item box. Pick the option “Change Appearance” > “Change Armor Pigment”. Now you can freely pick a color of your choosing. You can color individual gear pieces or color them all at once.
Important: You can only change the color of high-rank armor (6 stars+)! Low-Rank armor cannot be colored. The only place to make the change is in the item box inside your room, it doesn’t work from other item boxes.
You can also change the colour from the forge armour screen by pressing the start button (on Xbox).
How do we get to change our armor pigment using the color slider? I only see an option for color presets…