In the various chat-channels, with the exception of private messages, you can change the colour of your text by placing a six digit hexadecimal string encased by brackets infront of the text.
[FFFF00]Hello! will render as
Only the text after the string will be changed, so:
First part, [FF00FF]Second Part. will render as
First part, Second Part.
There is no limit to the number of colours, so:
First part, [FF00FF]Second Part.[FF1350]Third Part. will render as
First part, Second Part.Third Part.