Step 1- Go to image then mode, and click on cymk, on the right hand side the colour mode boxes will apear which include the colours.
Step 2- Choose the colour you want to keep, i have chosen Magenta & Yellow. You can also adjust the image by clicking on image, then go to adjustments then click on curves, i made my image slightly darker to improve shadows.
Step 3- Once you have adjusted the curves in your image. press okay, then go back and click on CYMK in the bottom right corner, and your image will go back to its origional, but it will be darker.