I use Aseprite, but you can use any image editor with an eye dropper and fill bucket. GraphicsGale is free.
Copy the tree and ground tile from the color you want, and paste it into an empty space on the tree wall. Use the eye dropper to grab one new color, then switch to the fill bucket and tap on that equivalent pixel in the tree wall (make sure your full bucket is filling non-contiguous pixels, or use the magic wand then fill the selection). Repeat for every color (there are maybe 16-24 unique colors).