Need experienced modder to help me fix color problem.

I have an issue that I cannot fix and I need someone with modding experience to help me.

I have a problem where every time I convert a bmp file in the turtle pack Slp editor the black areas become slightly lighter.

First I edit the picture in paint, then I save it in GIMP 2 as a bmp file, then I convert it in the slp editor which is where the change occurs.

I am getting stressed out over this and need someone's help.

Left is the black before it changes, right is after, it becomes slightly lighter