06-06-2020, 07:21 PM
On an image like this with dark lines on a white backrgound,
- to color the lines, you can paint after setting the tool to Lighten only mode. Since white is lighter than anything else, the white remains white. An equivalent way is to add a layer above this, set it to Lighten only mode, and paint on it in Normal mode.
- likewise to color the backrground, you set the mode to Darken only:
In the general case, though, you would transform your image into lines over transparent background by using Color>Color to alpha, and either paint the lines after alpha-locking the layer, or paint a background using Behind mode (or another layer under your lines). This will preserve the smooth edges of the lines.