Well theres 2 options you can either download an MOD that one of the people here have made themselves, or you can just use an imaging program like photoshop to change the color, its hard to give you a step by step without knowing what program you might have, you cant use the regular paint on the computer, because it doesnt preserve the transparency of the image, so youd end up with a big white box around it,

If you want to download an MOD from one of the other players they can be found Here instructions are usually part of the download but if your still not sure just PM the person who made the MOD you chose.