I decided since I had the room, to add a "dummy" color to stand in to the first color that was being black. The palette showed the dummy color as the first one. The dummy color was a hot pink, which I used to color the tree trunks. Then I made sure the original tree trunk color was still in the picture (or else it wouldn't show up in the palette), I put it it to the extreme left, and it doesn't show up in the game. I changed the colors to 4-bit and changed the hot pink to a different shade of brown. As a result, there's 11 colors in the palette, and the first one is black, but since it's not showing up in the game, there is no black color in that part (except for the text, but that's different.) Here is the result:

While I was at it, I changed the colors of the grass blades down at the bottom to simplify them. They were showing up as 4 or more different colors. So I made them 1 darker color than the other grass color. Who knew it would take so much work to do this? I also sped up the games because I found them to be slow.