You'll need to get a bit technical with this! In your text box, simply put: 

<color=red>

<color=blue>

<color=yellow>

<color=green>

<color=purple>

Then type whatever you would like to say! 

If you would like to write in any shade of colour you would like, you can visit this site, Choose the colour you would like, and use the HEX code like this: <#DF7886> then type what you would like to say.