RGB text like sonoyuncu & craftrise but in vanilla(1.17+). Works with chat messages, titles, scoreboards, tab list.
copy the files under the directory core into your resourcepacks assets/minecraft/shaders/core directory and you should be good to go. To make the text rainbow, make the color of the text #FEFEFE(rgb values are 254,254,254) f.e. the command /tellraw @p {"text":"test","color":"#fefefe"} will give you a rainbow text