Has anybody an idea, how to make a very long text colourfull ???

Actually i don't get on what you're aiming at.
Btw: It's colourful

well ... i have a VERRRY long text. e.g. a Word Document. And I want the letters to be colourful. Green, yellow, pink whatever ... every letter with a different colour ...

In Word you can do a find and replace and add formats.

Go to more on the replace pop up. Then click format. You will have to do a replace for each character so probably 26 times if you have lots of text.

or just record a macro.