It's been two years since this was asked, but to anyone who stumbles on this question:
You can print colored text using ANSI escape characters. However you need to literally print the escape character because printing "/e" will not work. You can copy and paste the character and store it on a string.
If you are using Windows you will also need to change a system register to allow Godot to use the console ANSI interpreter:
HKEY_CURRENT_USER > Console > VirtualTerminalLevel to 0x1.
I made a plugin to help with that: