Adjusting an Image

I found this image to adjust on Photoshop. I decided to make the image black and white, leaving one colour. I chose blue to keep. 

First, I duplicated the layer and made the background black and white. I then selected the blue parts on the second, coloured layer. I copy and pasted this and hid the second layer so that only the blue was in colour. Using the eraser tool, I tidied up the edges. 

This was the end result:

Here is another example. I used the same process, except this time, I kept the red in colour instead of the blue.