Multicolor version
Rollover the image below to see the before and after effect.

100% crop

Colorized Version
Rollover the image below to see the before and after effect.

Image when printed

use the Median filter. Apply a Median filter (Filter> Noise> Median) with radius of 2 pixels.
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Select the Magic Wand tool (W) and apply these settings:
Tolerance: 8
Anti-Alias: Checked
Contiguous: Checked
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Use the Magic Wand tool to click on a white area. Inverse the selection (Select> Inverse or Shift+Ctrl+I)
Apply another Median filter (Filter> Noise> Median) but with a Radius of 3 pixels.
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Deselect (Select> Deselect or Ctrl+D) and apply an Auto Levels (Image> Adjustments> Auto Levels or Shift+Ctrl+L)
{mosimage}Invert the current layer (Image> Adjustments> Invert or Ctrl+I) {mosimage}Duplicate the current layer (Layer> Duplicate Layer or Ctrl+J), change the blending mode of the new layer to Overlay, then Merge the layers (Layer> Merge Layers or Ctrl+E).