You can type rainbow text messages in the chat, in vanilla minecraft!

https://imgur.com/AHUsMcD
https://imgur.com/VUI0cAH

You must be on any version higher than 1.16 !
Latest version tested: 1.19.1
You require to be op and either have the server console or the server has to have enabled command blocks because the commands can be very long and the chat box will not be enough :)

1. Open the program
   
2. Type your in-game username and hit enter (spaces are ignored so you can have as many as you want, they will be stripped out, as well as every character after the 32th)
   
3. Type the message you want to type and hit enter (For now only ASCII is supported, so Cyrilic, Chinese and special symbols will not work!)
   
4. And you are done! The command will be automatically copied to your clipboard so you just have to press Ctrl+V in minecraft console or command block. If the command is not copied (example: you dont have enough memory) the command will be written out in the console)
   

You can download the program in the Releases section