Just used the app for a few minutes, first of all - the positives:
1) Has a lot of potential
2) Incorporation of the RGB colour pallette was a smart decision
3) The addition of the swatches is pretty good idea
The negatives & feedback
1) You need a mini tutorial, as the UX is too simple - If you are going to appeal to users on mass you need an re-loadable tutorial via settings (ellipsis)
2) The full screen adverts are annoying, may be an iap option to remove them?
3) When clicking on the circle it is not evident in the preview in the right hand bottom corner of the RGB pallette which one you are editing.
4) When you click the second circle, it loads the same RGB pallette location as the first circle
5) The ability to add custom hex codes in the RGB pallette would be brilliant
6) Bring up the swatches pallette, save, hexagon toggle, photo filter from the ellipsis menu or if you want add settings that enable this thus giving the users the option if they want to disable or not - I wouldn't recommend this approach is its unnecessary code and most people wouldn't disable it. I'm sure you could strip off a pixels of the top and bottom off the preview screen to make them fit + nudge it up abit and use the dead space at the top.
7) Constantly tapping the elipsis, then choosing the same photo to adjust the opacity is a no go. It will get old pretty quick - you need a opacity adjustment slider + opacity toggle. Said toggle would reside in the app bar, so if I want to adjust the opacity, I will tap that toggle and to bring up the opacity slider, then tap the image to confirm or the done button.
8) The back stack needs work, for example after choosing a photo I decide that photo won't work, I press back - the app closes - it should go back to the photo hub - the same applies to the swatches pallette, rgb pallette, photo picker etc. Lastly Prompt the user if they want to close the app not close it automatically.
Additional features:
1) Incorporate as a camera lens
2) Add a camera function directly within the app
Questions:
1) When setting the image as lock screen background, the dialogue box says it may update it periodically. Does the app need to periodically update images?
As, your only customising one image or does this form part of feature that you have in mind?
2) How does the swatches pallete work? It appears to be hit and miss.