Map tileset not working correctly due to the colors used

I’m trying to insert a map tileset and seems to work but after I close the tileset window, the tileset gets destroyed

how it looks normally

How it looks after closing

I’ve been told that the first tile of the image at the top left corner should be empty but I tried already that by using a new color but it didn’t worked and using the color reduction tool didn’t worked either. Does someone know what I should do to fix this?