Haha, when I started making my own patterns from images, I use FlossCross.com which has the option to export as an fcjson, then I wrote a script to convert it into an Excel doc.
Excel is able to lookup cells by background colour, so I'd find cells with the thread index with the original colour (as in, not ones I've marked as green) in order to find remaining stitches per colour.
Obviously it's a bit clunky so I made ClickStitch!
Woops you ain't wrong hahah, guess I'll be seeing grey for a while 😅