When placing a sign on the north or south side of a block, the text will appear very slightly dimmer than if the sign is placed on the east or west side. This is most noticeable with white or other bright colors.

HOW TO RECREATE:

1.) Put some signs on different sides of a block and then use a bright color to dye the text.

2.) Notice how north and south facing signs are slightly dimmer than east and west.

See attached screenshot.