How do I change a specific colour in a specific layer?

[Ofnuts]

The best way is to extract the lines from the background: in other words, have the lines on a transparent layer, and the backrgound layer under it. This lets you have partially transparent pixels on the edges of the lines (that re important for smooth lines).

The you set the alpha-lock on the lines layer so that painting will keep the opacity of the pixels, and these 'smoothing" pixels will receive the new color but remain partially transparent.

If your lines are currently on a uniform background, you can extract them using "Color>Color to alpha" to make the background transparent.

For more information see: https://www.gimp-forum.net/Thread-Backgr...r-graphics