In this tutorial I am going to quickly change a normal image to a montone one. First of all open the image you want to change into a monotone one. I am going to use the car image again from the first tutorial.
A dialog box will open asking for you to choose the new layer properties. Leave everything as it is except in "Layer fill type" choose foreground color.
Go to layer modes and select "Color".
Right-click on the first layer and select "Merge down".
Now here's how your final image should look like. Wasn't that easy? Well that all for this tutorial.
UPDATE
I wanted to add this but forgot it when making the tutorial. When I told you go to layer modes and select "Color" you would have noticed that there were many other options too. Select them all and see what happens. If you want to understand more about this I recommend you go here.