Signs have constant ligh levels, only depending on which direction they are facing, independent of any source of light (sky or block).

This makes it really hard to read white text with the default textures when the sign is lighter (facing North/South) and can be easily seen when it's darker (facing East/West).

.

The sign used in the image attached was obtained using this command (in vanilla 1.12.2)

/give @p minecraft:sign 1 0 {BlockEntityTag:{Text1:"

{\"text\":\"Test\",\"color\":\"dark_aqua\"}

",Text2:"

{\"text\":\"Test\",\"color\":\"red\"}

",Text3:"

{\"text\":\"Test\",\"color\":\"dark_blue\"}

",Text4:"

{\"text\":\"Test\",\"color\":\"white\"}

"}}