If you can't even envisage the problem, I'm struggling to understand why you're trying to make an app to fix it. I feel like you're approaching the development of this whole system backwards, by deciding you want to make a solution for something that people will buy, and then hunting for a problem that solution will potentially apply to...
But okay, this is what one of my pages looks like before I colour it:
And this is what it looks like after:
I use a rough palette like this to keep track of which characters have what colours on them, and update this every time the characters get new outfits (though one-off outfits in flashbacks or on minor characters I just deal with as they come)
So... the reason I can't just get a computer to do it, is because the computer does not know who Rekki Lune is. The computer might recognise that there are drawings of people here, and might apply skintones to them when it detects a face, for example, based on machine learning, but it won't know to spot my characters specifically and apply the right colous to the right areas so that each panel has those characers coloured consistently; ie, so Rekki's pants are red, her skintone is that specific colour etc.
Since most of these areas are flat fills, a large part of the process is just clickng an area to fill it...meaning that a tool where I select a colour and click that area to "auto colour" it saves me no time. It might save time if I was doing some sort of soft-shaded style, or if I was making a comic where for stylistic reasons, consistency wasn't an issue (like say if it was in the style of "V For Vendetta"), but for me, it won't. I'd need to keep correcting its choices about colours, like "no, Rekki's skin needs to be this colour," "no, this part you sensed as being skin is actually tights..." It won't save me time.
If your goal is to make a product to sell to comic artists... which er... may be a bad choice of market, we're not the richest bunch of people.... You really need to sit down with some and do proper UX research; watch streams of webcomickers at work, tutorial videos showing processes, look for pain points.