When the feature where you could use dye on a sign to change the color of its text was implemented, using the section symbol (§) would also bring these colors, instead of the ones that you can use in Command Blocks and Chat (and you used to be able to use them in signs). Upon further research, there is a tag that I have found (using Universal Minecraft Editor) called "IgnoreLighting" (if you use the +DATA NBT on a sign) that will be set to 0. Setting it to 1 will fix this issue, but it shouldn't have to be this way.