Duotone

I have been playing around in Photoshop, like I tend to do in my spare time and am spending a fair bit of time trying to learn the ins and outs of a feature called Duotones. We were shown it in the first class of the second Photoshop course offered at Algonquin and I was really interested in it. Basically, you take a grayscale image and add inks to it one at a time. You adjust the intensity of the inks you apply the same way you would adjust the lighting via curves so that part of it is familiar at least. You can have one ink (monotone), two inks (duotone), three inks (tritone) or four inks (quadtone) applied to an image.The first image here is one I took way back in 2008 while in London, England. It is much more subtle then the below image from Chicago, taken last year. They are both quadtones (four inks applied) but obviously have different colours and intensities applied. The Easter Island image has three grays and a pink while the Chicago image has a black, blue, purple and pink applied in that order.
When using the Duotone feature you have to keep in mind that it is best to apply the colours from darkest to lightest. The logic wasn't explained to me by the instructor, just the rule. But it makes sense as the inks are applied almost like screens. If you place a darker screen over a lighter one you won't see the lighter colour at all.
I am far from perfecting duotones, as you can clearly see in the images I have done so far, but I like the effect it has and how it can range from subtle to in-your-face so easily. This is definitely something I am going to continue working with.