Made in PSCS; Involves Selective Coloring.
Start by cropping your base, sharpening it, etc.
Here is my base, picture from winchesterbros.com:
Duplicate your base and set it to multiply:
Duplicate your base once again and set it to screen. Make sure that the screen layer is on top:
Layer > New Adjustment Layer > Hue/Saturation. Up the saturation by twenty.
screen cap Layer > New Adjustment Layer > Selective Color.
Reds:
Cyan: -50
Magenta: 0
Yellow: +20
Black: +15
screen cap Yellows:
Cyan: +15
Magenta: 0
Yellow: -20
Black: -15
screen cap Neutrals:
Cyan: +15
Magenta: -9
Yellow: -15
Black: -15
screen cap Other icons made with this coloring:
If you need something explained, or if something was confusing, please let me know. :)