For some reason, when coloring the text using dye, it just get darker than usual, and so I tried the Command block version

/give @p minecraft:spruce_sign{BlockEntityTag:{Text1:'{"text":"THIS IS WHITE","color":"white"}'},display:{Name:'{"text":"Custom Sign"}'}}

It actually gave me the real brightness of the coded color of the game, and later then, I compared the dark gray dye with black dye, for some reason they just looked the same.

Thanks in advance if ever this report is noticed.