Yeah, just how do you make a mod to change the color of the game terrain, water, and have different trees? I don't know where to ask or where a tutorial is for this, but if anyone could help, it would be nice.
I recommend you to learn the basics of the Reader and SC4tool (if you don't already know them), then once you know how to mod things a bit, look to an existing mod to see how to do it. Its not a really easy process, so I think you should do that, then when you have some specific questions, you could come back to ask them.