I had trouble mixing paints to match colours in my reference photos so I wrote myself a computer program to help. I can load a photo of my paints to tell it what I have available and photos of my mixes so it can tell me how far off I am and what to add next.

I added code to generate a sketch from the reference, scale it to my canvas and print it for tracing over multiple sheets with markers to line them all up properly.

There's code to break the reference image into areas of colour close to my available paints.

You can load photos of your work-in-progress and it can compare them with your reference and suggest changes.

No one but me has used it yet and I'd love it if someone could look at the web site and give it a go.

You can find it at https://www.artistshelper.com/p/lemmy.html

It'll be full of bugs to start with but if you report 'em I'll try to fix 'em.