A quick note on usage of PNG: the formats of X-Plane and PNG images are not quite the same.
- To provide a non-alpha image in X-Plane, use an RGB or grayscale PNG.
- To provide an image with alpha in X-Plane, use an RGB or grayscale PNG with an alpha channel.
- To provide an alpha mask in X-Plane, use a grayscale PNG.
Also please note tht you can use a color or grayscale PNG for a color texture, but internally X-Plane always operates in RGB, so you don't save any VRAM by reducing your PNGs to grayscale. You might save some space on disk.