Many of the new compact cameras have an interesting gadget included: A colour key filter. You choose one of six or so predefined colours and the result is then a black and white picture except everything in this colour. However, the colour is very narrowly defined. If you choose green, there is a medium green, much lighter or darker greens stay black/white.

I am looking for a filter or a plug-in for Gimp, doing exactly this, but where I can set the borders of the colours myself. The best would probably be a method to define one (or more) sets of colour key ranges per colour.

I found a tut for Photoshop doing at the bottom exactly what I would like to have, however it seems to me to be very complicated and not easy to use the technique in Gimp.

I have scanned the Gmic filters, but found nothing usable.

Has anybody heard of a way to do this or knows of such a filter/plug-in? My eternal thanks are sure.

The nearest thing is the "Select by color" tool, playing with the parameters.
Best results makes "select by:" 'composite'.
After selecting, enhance the selection by quick mask or whatever.
Then invert the selection and desaturate.

Hatti

If that is the very last image you want to replicate then look for a little application called Tintii - free if you use linux

In Gimp all sorts of ways, as previous post 2 layers, a selection & cut or use a layer mask.

For a change, this plugin multi-band-blending http://registry.gimp.org/node/31957 which can give a more subtle effect than a plain layer mask.



original on the left, tweaked on the right.

What you need, three layers, A desaturated copy, the original (tweaked or not), a b&w mask (easily made with the threshold tool & color inverted)

Hi and thanks very much for this explications. I tried try «tintii», which is even in the Repos of my Kubuntu. And the name of the app reminds me of my longstanding love with Tintin (the comic series from Belgiums Master Hergé). This love is live with me at least since 60 years, I still have all volumes, many of them in the first edition. In French, of course.

Anyway, thanks a lot for your big help.

Hello
After my first playing session, I have now tried a couple of more difficult situations. The following picture shows what I find could be improved by, the red along the crane to the top is rather disappointing, also the Sky has got a hint of blue. Perhaps a layer mask to stop colouring in places I would rather not like the colour. Also a bit more individually definable width of the colour bracket might be helpful.

This application is availabe as stand alone or as a Photoshop plug-in. It might well be, that this plug-in would answer positively to my wishes - is there someone here with a Ps who would like to check this?

I will contact the author of this really great idea with my hopes.

I found reasonable results with g'mic, using default values​​.

Sorry, i have not expressed myself correctly. I did NOT want those creeping colour lines up the crane, nor the blue sky. The Idea was just the highlighting of the traffic signs and the blue van.

Color select, with other than default selection criteria (look at Hue, or specific channels), then invert the selection and desaturate the rest? It could also be possible too do one first selection using the "Hue" criteria, then intersect it with another done with the "Saturation" criteria.

@Arran
You should get a good result with one of Gmic filters.
but just for interest, this is the first image (original) clipped out and run through the multi-band-blending (mbb) filter.



I am not saying it is the best way, a slightly blurred layer mask would be my usual way for something like this.

This is what the layers look like in Gimp, as required by the plugin, and a zoom of the result.



I did not realise you used a linux so here is mbb complied for both 32 and 64 bit.
Remember to check permissions.
The plugin requires all layers/images the same size and all to have the same alpha/no-alpha otherwise you get an error message.

edit: curses, missed the parking sign. Doing too many things at the same time here.