Changing the color is easy, you just have to extract the texture you want to edit and put it back again with your modifications. There are plenty of videos on youtube for that. Now adding shoulders, if you mean altering the model structure, that can't be taught easily, you need to be proficient in 3D modelling software. And you need to also have a paid tool that allows you to import/export unreal files from said 3D modelling software.
sepultribe's post in Customize Armors was marked as the answer