The newsprint filter is a type of 'artistic' halftoning. (For those who do not know Halftoning = dots on a regular grid, dots varying in size to give an impression of shading.)
A solid colour therefore is a grid of equal sized dots. You can do that manually. While it would be nice for the GEGL filter to do this, it does fail unless there is some variation in the image.
This an example with some gradients introduced next to solid colour: https://i.imgur.com/MtGHTMq.png
There are other ways, as denzjos, the gimp_gmic_qt plugin http://www.gmic.eu has some halftone filters
Another old plugin generates a greyscale image. example: https://i.imgur.com/kCBczm1.png For colour it can be used as an overlay.
You can get that from: https://gimplearn.net/viewtopic.php?t=190
edit: search for alternatives
1) The old (Gimp 2.8) Newsprint plugin. You said you had old projects where it worked, maybe with the old plugin. It does work with Gimp 2.10. If you are using Windows I can provide, linux probably provide (depends on distro) . MacOS - no.
Gives this: https://i.imgur.com/6OR8wq4.jpg
2) In Filters -> Render -> Pattern the unlikely Linear Sinusoid. Looks like this. Note the mode from the Blending Optionsdrop-down menu. https://i.imgur.com/SL77fYo.jpg
A solid colour therefore is a grid of equal sized dots. You can do that manually. While it would be nice for the GEGL filter to do this, it does fail unless there is some variation in the image.
This an example with some gradients introduced next to solid colour: https://i.imgur.com/MtGHTMq.png
There are other ways, as denzjos, the gimp_gmic_qt plugin http://www.gmic.eu has some halftone filters
Another old plugin generates a greyscale image. example: https://i.imgur.com/kCBczm1.png For colour it can be used as an overlay.
You can get that from: https://gimplearn.net/viewtopic.php?t=190
edit: search for alternatives
1) The old (Gimp 2.8) Newsprint plugin. You said you had old projects where it worked, maybe with the old plugin. It does work with Gimp 2.10. If you are using Windows I can provide, linux probably provide (depends on distro) . MacOS - no.
Gives this: https://i.imgur.com/6OR8wq4.jpg
2) In Filters -> Render -> Pattern the unlikely Linear Sinusoid. Looks like this. Note the mode from the Blending Optionsdrop-down menu. https://i.imgur.com/SL77fYo.jpg