I have seen such an effect being used in many places however I haven't figured out how to replicate it via Photoshop.
I have tried desaturating the image, placing it under a layer of solid colour and then lowering the opacity of the colour. I have also tried a couple of blend modes, to no avail. The effect is nowhere near the same.
Can someone please tell me the effect used on these images?
This is most probably a Gradient Map. Here's a quick how-to:
Gradient Map adjustment layer (Layer > New Adjustment
Layer > Gradient Map... or use the black/white circle icon in the
Layers palette)This will render your image in just the colours in a gradient of the two chosen colours, with the background colour matching the brightest parts of the image, and the foreground the darkest.
If you're not happy with the result, open the Properties palette (Window > Properties), it will show you the gradient used and wil allow you to edit it by clicking on it. Since this is the standard gradient dialog, you can even pick a stock gradient or one you saved earlier.
I'd suggest using the Multiply blend mode, with the image in front of the color layer. (Technically, with Multiply, the layer order doesn't matter, but having the image layer in front lets you adjust its opacity instead of having to tweak its color levels directly.)
For example, here's my quick attempt to recreate the NYC image. I don't have Photoshop so I'm using GIMP, but the steps should be basically the same in either program:
Start with a suitable photo. For this example, I'm using this freely licensed (CC-By-SA 2.5) panorama of Manhattan by AngMoKio at Wikimedia Commons:
The first step in the process is to convert the image to grayscale. For maximum control over the results, you'll want to use the Channel Mixer. For example, here I mainly took the red channel and a bit of the blue, with the green channel mostly ignored:
Now, create a solid color layer (I used RGB #01baff = HSV (196, 100, 100)), move it below the grayscale image layer, and set the image layer's blend mode to Multiply. The result should look something like this:
We've already got a nice Duotone effect, but there's way too much contrast for what we want. To fix that, just reduce the image layer's opacity to, say, 25%:
This is already pretty close to what we want. To tweak it a bit more, I adjusted the grayscale image layer's color curve (pulling up the midtones to add more contrast in the shadows, and adding a nice smooth curve at the top to smooth out the highlights), reduced the opacity to 20% to compensate, and added an extra black layer with 7% opacity to darken the (now slightly too bright) background a bit:
And here's the final result:
(It's arguable whether the tweaks in the last step actually improved the result or not, but I included them anyway just to show that you can adjust the contrast and lightness like that.)
Use Image > Adjustments > Hue and Saturation... Desaturate the image by moving the slider for Saturation all the way to the left. This removes the color.
In the layers panel, Click on the tiny "fx" icon at the bottom of the layers panel and choose Color Overlay. In the Effect dialog box that appears, click on the color box (it's usually red by default) to bring up the color picker and choose the color you want to use.
Move the opacity slider down to 50% or whatever setting gives you the effect you want.
If the desaturating causes the image to be very gray, you can use Image > Adjustments > Auto Levels, or adjust it manually with Levels to bring the sliders into line with the information in the histogram.
