ryrona
This is why GrapheneOS sets an all black background image.
No, it isn't why we set an all black background image. We use that because it fits the aesthetics of GrapheneOS and many people like it that way. Leaving it at the default will avoid it being useful for fingerprinting regardless of what's used as the default wallpaper. Having fewer people want to change it to something else does reduce the usefulness for fingerprinting but it isn't why we chose it. Choosing a default that's appreciated by a higher number of people is one of the main factors in choosing the default regardless.
In theory it should be possible to patch away this fingerprinting vector, but it seems GrapheneOS has not done that, at least yet.
There are a huge number of ways to do fingerprinting and this really isn't needed. There's very little point in eliminating specific methods based on media coverage while not doing anything significant to move the needle overall. Removing access to custom wallpapers and the color scheme derived from them is on the roadmap of things we can do in the future but it's an incredibly low priority and has no actual value by itself without a massive number of other changes.