Seems that it is somewhat difficult to find a simple solid color wallpaper for an Android tablet. I just bought the ASUS ZenPad and I was unable to find a simple one color wallpaper to replace the default wallpaper. In looking for how to replace the default wallpaper, it appeared that many others also had the same frustration. Given that, one would think that having a no wallpaper option would be easily available. Well it wasn't.
After doing some searching, I was able to find a plain black wallpaper at this website: Black Solid Color Background. This website also offers other solid colors. Of course I need to add the disclaimer that what worked for me, might not work for you.