When applying white dye to a sign, the color is different based on the direction the sign is facing. White dye appears to be grey when the sign is facing north / south (seen via F3)

The first time I saw this, it was written on a spruce sign, but it appears it happens with all the different types of signs.

To test: create a new world (with creative mode enabled) and place two signs (against a block or on the ground, doesn't matter), one facing north / south and one facing east / west. Type some text and dye both signs with white dye. The result is the sign facing north / south is colored grey and the east / west facing one is colored white.