The colors in writings on the signs of change contrast according faces.
example:

• A sign placed on a block that faces the EAST or WEST, scriptures will be clearer.
• A sign placed on a block that faces the SOUTH or NORTH scriptures will darker.

Here are some photos:
Facing the East - http://i.imgur.com/t0kzltv.png
Facing the South - http://i.imgur.com/ROkV33M.png

I'm so sorry for my very bad English ^^ '
I used google trad: x