I love that effect where you see a single colour in a black and white photograph. I saw an example again yesterday and decided it was time to learn how to achieve it.
I used Photoshop to manipulate the image. I don't think I could do it with my free version of Photoplus (V8) but if you know different please let me know.
After a few dead end experiments, I finally came across this method of using the colour replace feature. Not absolutely workable for all images as you will see below but good enough.
The photo was not particularly brilliant but does show the effect. So I tried it out on a couple of other images to make sure I could do it properly.
I liked the example with the fuchsia, it did seem to work quite well. The example with the tulips did show the one issue with this method. I could not change the red foliage in the background. Maybe with a lot of tweaking it would be possible but in trying to select the red foliage the tulips were always partly selected. However to try and make it easy I had selected a high fuzzy value, this decides how broad the range of selected colours will be. So doing it with a low fuzzy value should be the answer. I will experiment and let you know how I get on.
So what did I do?
With the image in the active window, select image / adjustments / replace colour.
I set the Fuzziness as indicated above to about 170.
Select the colour picker, on the left of the 3 dropper icons.
now click on the image to select the colour to replace. If you select the middle icon you can add other colour values near to the initial colour .
Now reduce saturation as much as required, and adjust lightness if required.
Repeat this for any colours you need to desaturate.
You should now have an image with just the colour you wish to preserve in a black and white background.
I love this software I am experimenting and learning all the time. Lots of clever tricks have tutorials written for them but I need to know the simple effects that are possible. Walk before you can run is my motto.