warpstorm
Yumbo? Yumbo!
At the very least, you will need to blend it in with any file with an O in its name in the art/terrain directory. If you end up changing the coastal you will also need to change all the ones with a C in the name. If you are just changing the tint a bit, it shouldn't be hard. If you are making more substantive changes, be prepared for a lot of work. Making sure tiles blend correctly is the most time consuming part about making a terrain mod. Each tile has to blend seeemlessly with dozens of tiles.