This bug has been reported numerous times and was closed as "Working as intended" each time.

Example: MC-33304

Please take a look at the screenshot I have attached. In case you can't read it, which you'll probably have a hard time doing because of this very bug, it says:

"Please care to explain how not being to read this sign at all is considered "working as intended" behavior. ???"

Erik Broes commented on the previously closed issue that players are responsible for choosing a color that is readable because we chose to use a colored sign.
Quote: "The reason is because you are using colors. Because you are you are also responsible for picking colors you can see well."
Picking a color other than black is virtually impossible because of this bug, because it affects all colors other than black, not just the one I used in my screenshot.

I don't understand why you would force the player to use the default black color on signs on 2 out of only 4 possible directions? Especially since colored signs are now possible to create in Vanilla Minecraft through JSON formatting, it's not right that the player's creativity is restricted because of a bug in the game.

Please reconsider this, Erik Broes. Thanks for reading