So I resort to Photoshop and Lightroom. First I want to have a shot at a lego image.
In Lightroom the process seems to be to remove the saturation of all colours except the one you want to keep. In the lego photo here I reduced the saturation of all the lego colours to 0 except the blue. This still left me with some purple for some reason so I changed the hue of purple to blue - hence the multiple colours of blue in image 1.
I wish I had glasses that would actually change the colours - much easier to help the kids find their lego pieces!
The next image is not as bright as the lego one. I was interested in the orange around the face in this one and the almost mask like colour over the eyes. For this I tried out the steps that are followed in Photoshop. I followed these instructions.
Click on the background layer and using the key combination Control Shift U to desaturate the whole layer and turn it to black and white.
Then click on the duplicate layer and using the Select then Colour Range menu options you can use the eye dropper to select the colours that you want to keep.
Making sure you are still on the duplicate layer then click delete. All of the image will disappear except the colour you have selected. With the background layer showing through as black and white you will have your new image with one highlight colour.
I left a bit of orange in the leaves but maybe this detracts from the face highlight. I'll leave it for now.