I noticed today on a friend's bedrock realms plus server that, signs made before v1.16.220 release easily accepts white dye I was using.

However, it appears there is a bug where any new signs made now, if dyed, only changes the text to a grey color. When I tested, the game registered I used one white dye and inventory count went down by one, but it didn't show the text in white, only grey color. I tried this with red dye too and with same results.