This is a feature I'd love to see, especially for changing colored linework!

For example, I've outlined the arms on one of my drawings, and the lines are in blue! (My bad but...) If there was a color replacer, I could easily switch out the blue to a darker color close to the Italian/Korean she is. making my shading I'm about to do much more... conformed!

Sounds a lot like you want to use the "Colorize" option in the HSV Adjustment filter. Here is a short video demonstration: https://youtu.be/GNXDgN85tXU

I hoped this helped and kind regards

You can also use the similar color selector! It looks like an eyedropper with a bunch of selection ants near it! You can set the fuzziness so it selects similar colors, hope that helps

Another way if your lineart is over a transparent layer, is to preserve the transparent areas while you paint or fill the opaque areas.

To do so, just double click over the checkered alpha icon, right next to the lineart layer in the layerbox, and fill or paint with any color as desired.

or you could use the color replacer inside g'mic