Dark oak sign and other darker signs displaying black text rather than white.
Comments 2
Thank you for your report!
However, this issue is Working as Intended.
This is the same behavior as in Java Edition. When non-oak signs were introduced, the text was set to white, but there were complaints that white was too bright in some contexts and different people wanted different colors to be substituted. So Mojang decided to make it black and leave it to each player to decide what color they want their text to be.
You can control the color of the text using formatting codes.
You can use §f to make text white and §0 to make it black again
https://minecraft.gamepedia.com/Formatting_codes