If a Sign with color is created, the colored Signs text appears darker.

Used Commands:

North:
setblock ~ ~2 ~ minecraft:wall_sign 3 replace {Text1:"

{text:\"Text\",color:dark_green}

"}

East:
setblock ~ ~2 ~ minecraft:wall_sign 4 replace {Text1:"

{text:\"Text\",color:dark_green}

"} EAST

South:
setblock ~ ~2 ~ minecraft:wall_sign 2 replace {Text1:"

{text:\"Text\",color:dark_green}

"} SOUTH

West:
setblock ~ ~2 ~ minecraft:wall_sign 5 replace {Text1:"

{text:\"Text\",color:dark_green}

"} WEST