When looking at signs on 1.21.90 with Vibrant Visuals enabled, the text on both normal and hanging signs is black, even if the sign has been dyed a different colour. Click onto the sign (eg like if you were to edit the text) though, and the text is the colour it was dyed to.
Environment
•Open up Minecraft Bedrock Edition, in the 1.21.90 update
•Open 'Settings' and head down to your video settings
•Scroll towards the bottom of your video settings, and enable 'Vibrant Visuals'
•Come back out of the settings and head to the menu, in order to load up a world
•Once loaded into the world, place a sign (either normal or hanging signs, with either being of any wood type). For this example, I am using a Dark Oak Sign
•Pick a dye of your choice. Again, for example, I am using a White Dye
•Type some text onto the sign. For this example, I typed 'Text'
•Exit the sign interface, and dye it using the white dye
•The text should remain black, even though it has been dyed white, for this example
•Click onto the sign interface, as if you were about to edit its text. The text on the interface should be white (or the colour you chose), whilst the text when just looking at the sign remains black
This issue is being tracked at MCPE-218703.