Next, I go to the Image menu and change the Mode to Indexed Color. This allows me to limit the amount of colors within the image. I set the Palette to Local (Perceptual), set Forced to Black and White, and turn off the Dither. Then I determine how many colors the image needs in order to create an interesting texture. For this image, 7 colors separates most of the scratches from the rest of the photograph. After clicking the OK button, I switch the Mode back to RGB. This doesn't change the color palette but just allows me to work in the RGB mode again.
Next, I choose the Magic Wand and uncheck the Anti-alias box and uncheck the Contiguous box. Then I click on the color of the texture I want to capture, in this image it's the tan color making up most of the scratch marks. With the Contiguous box unchecked, the Magic wand captures all of the tan rather than just one contiguous section. I then copy and paste these tan pixels onto a new layer and hide the original image layer. I delete any recognizable or large areas of texture, in this case erasing Lincoln's hands, head, and shirt as well as some of the larger texture bits.
Finally, I copy and paste the texture bits to thicken them up, depending on how thick I want the texture to be. I rotate the pasted texture and move these pasted areas around to make the texture appear more random. I also rotate by 90º increments in order to avoid creating any anti-alias pixels. I lock the transparent pixels on this texture layer in order to quickly change its color. Locking the transparent pixels allows me to quickly change the color of the texture by holding down the Option key and hitting Delete (to fill with the Foreground color) or holding down the Command key and hitting Delete (to fill with the Background color). You can also use the Edit>Fill command or use the Pencil or Brush tool for this step. Since the transparent pixels are locked, no new pixels can be created so painting on the layer will only change the color of the existing pixels (but keep in mind that if you use the Eraser tool, rather than erasing pixels, you will be painting them with the Background color).
So that's it, pretty simple actually once you know the steps.