If you enable the checkbox Show whitespace characters under General → Editors → Text Editors, the characters are a bit too offensive. Sadly, there’s no option to change the transparency of them. The default (and fixed) value is 64h/FFh = 100d/255d (ca. 40% visible / 60% transparency). You can only change the value using a hex-editor:

• open the file org.eclipse.jface.text_3.3.1r331_v20070629.jar in the plugins folder of Eclipse using an archiver (e.g. Total Commander)
• unpack the contained file WhitespaceCharacterPainter.class out of /org/eclipse/jface/text/
• open that file in a hex-editor (e.g. Hex-Workshop, XVI32)

• jump to the location 0x1769 and find the following lines:

    001750: 0a 2a b4 01 1f b6 01 3c 36 0a 2a b4 01 1d 99 00
    001760: 21 2b b6 01 4e 36 0b 2b 10>64<b6 01 51 2a 2b 15
    001770: 09 15 0a b7 01 38 2b 15 0b b6 01 51 a7 00 0c 2a
  

  (The marked 64 is the value described above. 00h = white/transparent, FFh = black.)

33h would be 80% transparency and is a nice value.

After saving your changes, put the file back into the .jar and run Eclipse.