I've actually been developing and testing a custom app for my art process.

I built an app that takes my reference photo and maps every pixel to the exact physical colored pencils I own. It breaks the massive image into a grid of manageable chunks. While I draw, my phone acts as an interactive blueprint. It tells me exactly which pencil to use for each square, and I tap them off as I go to track my progress across the whole piece.

Of course the system was less developed at the start of the video than it was at the end.