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1. First off, create a Selective Coloring layer. To do this go Layer >> New Adjustment Layer >> Selective Color (pop-up image example). Once you've created the layer, pick "Reds" in the drop down menu above and input the numbers as follows:

Cyan: - 100
Magenta: + 100
Yellow: + 100
Black: + 100

2. Now its time for another Selective Coloring layer. You can just make one layer and input all the color settings into it, but this way you can mess with the different layer opacities and stuff. So set this later to "Yellows" in the drop down menu and input these numbers:

Cyan: - 100
Magenta: 0
Yellow: + 100
Black: 0

3. Our third Selective Coloring layer will be set to "Greens". Now put in these numbers:

Cyan: 0
Magenta: - 100
Yellows: + 100
Black: 0

4. The fourth Selective Coloring layer is set to "Cyans" with these inputs:

Cyan: + 100
Magenta: - 100
Yellows: 0
Black: 0

5. Now, dears, we will make a yet another Selective Coloring layer and set this one to "Whites" with these inputs:

Cyan: - 100
Magenta: - 100
Yellows: - 100
Black: - 100

6. Finally, it is time to make a Solid Color layer. To do this go Layer >> New Fill Layer >> Solid Color (pop-up image example). Fill this layer with #c4dedc (pop-up image example) and then set it to Soft Light with 100% Opacity (pop-up image example).

And now we are done! I encourage that you mess around with the Opacity of the Solid Color layer and the other layers if your image looks like it barfed and rolled over dead. Not all coloring techniques work right for every image.

Other icons made in the same technique at dif. levels + opacities: