Not a painter myself, but based on some playing around with the web app, I can certainly see how a painter can derive a lot of value from this. Seems comprehensive, yet practical and to the point.

Some feedback on the workflow. It seems like uploading a photo is optional, but it’s not all too clear. I’m assuming some artists might want to create a palette for something more abstract that doesn’t require a photo. Maybe focusing first on the palette, with an option of basing it on a photo might make more sense as a workflow. Then again, I’m not your target user.

Almost goes without saying, but being able to use it fully without registration will certainly get you all the HN points here.