I have a nice green dragon.

"Import PDF" image to Photoshop Elements
Choose "Enhance" "Adjust Brightness/Contrast" "Levels"
Pick the Blue channel.
Slide the LEFT slider about half way across (to the center)
OK and save.

Any image editor that lets you set the brightness/contrast independently on the 3 separate color channels should get the same effect.

Now as soon as the ink gets dry...

Martin Gardner is well known for his several books, which never fail to amaze. The "Puzzle Corner" he wrote, for about 30 years, for one of my magazines also showed him to have something of a sadistic streak - although he never failed to come up with elegant solutions to the problems he posed.

John