You know what I wanted in 8.1

[ToonU]

A color picker for the accent color. I hate the preset colors. At least they gave us an option to use an image

 

[Nimdock]

Color picker would have been nice. But you are correct, you can create an image of any color and make that your background.

The drawback of course is that the accent color is used on other places, not just for tiles.

Also, if you have a Windows 8.1 computer (not sure 8) and you have set the themes to sync I think you can set your theme in Windows 8 where you can pick whatever you want and have it show up in WP8.1.