I feel kinda bad writing the first part of this tutorial since naginis over at accios did an awesome job explaining this concept of using the brown standard gradient map to color icons. Be sure to check it out here for a different viewpoint. Because of the already great tutorial on the subject of muted yet vibrant coloring, I'll try to talk about different things but this may turn out to be more of a tutorial :)
I set the gaussian blurred layer to hard light @ 100%:
And then I added a gradient map!
After that, I duplicated the original screencap and set it to 51% opacity:
I then added a brightness/contrast layer (brightness: 25, contrast: 23)
So, when I made the first parts of the icon, the screencap was a larger size and when I resized the image, that became cropped :( So, here is the original crop :) Either way, after the brightness/contrast layer, I added a surface blur (IDK why - I thought it looked cool at that moment) and sharpened the icon for the final result!
+ sharpening --->
I then darkenned the cap using curves to increase the contrast:
Then, I increased the brightness/contrast:
I added a gradient map and set it to hard light @100
I then added one more curves layer to brighten up the icon to be able to see their faces :)
Here I increased the sharpness and added a couple of textures to complete the icon!
Basically with gradient maps/gradient overlays, it's all about experimenting and learning about what colors go well with the picked screencap. And the really awesome thing about gradient maps is that you can just add them and call it a day. I typically edit the cap after adding a gradient map so I can get the look I am going for though and I reccomend that you do the same.
In this image, I used a gradient map and set it to hard light to tint Diana's face purple and add more blue-purple shades into the image.
In this icon, I used one gradient map in the background and then layered one more gradient map on top of Sherlock and erased the parts around him in the background. You can check out the gif to see how I got to the final image.
Example of Gradient Maps Used in Icons
Some of these are assumptions but these icons look like they've been made with gradient maps :)