Also notice that with 8.1 you have the option of using your desktop wallpaper as your start screen background. This isn't as powerful as Decor8 options but considering that Decor8 no longer will handle the Tiles background it negates the necessity for Decor8 at all anymore. Sorry to say that I've removed it. This was a great asset on Win 8 but no longer in it's current state under 8.1.
Another note concerning Modern Mix on 8.1. - My "Store" app kept popping up at random for no reason at all. I reset the store cache and also checked if there were any updates needed. Neither of those issues addressed the issue. I removed MM and so far no issues.
Regarding Decor8, any complaints regarding the tile colours would need to go to Microsoft as they actively broke that capability. It is true that they used to look a little plain all being the same colour, but having colours you cannot change at all is less than ideal I agree.
There is one major advantage of Decor8 "use wallpaper setting" over the Windows 8.1 wallpaper setting which is Decor8 automatically creates a matching colour scheme vs grey. Depending on your wallpaper this can really add to the general feel. That's not to say the Windows 8.1 option doesn't have some nice points too and this may become more apparent in the future. You are limited to the same background as your wallpaper of course.
On ModernMix, can you think of anything you were doing prior to the Store opening? ModernMix 1.05 would show the store, but 1.1 should have resolved this.
We do plan updates of most of the Windows 8 apps for Windows 8.1 ready for the public release of Windows 8.1 in a month or so. We have Start8, ModernMix, Decor8 and DeskScapes updates in the pipeline right now.