How can I change the color of the text/font for my emails on iPad?

I do not know if there is a direct way to do it, but there is a work-around, namely to use Numbers, Pages or Keynote. In those apps, it is straightforward to type text in other formats, such as text color, highlight (in yellow, for example), and then copy and paste into the body of your email. I hope that helps.

on Mail (the native app from Apple) you can only choose between bold, italic and underline.

Unfortunately there's not for a while an option to change color or font size.

Afaik you can't. Perhaps a third-party email app like Outlook can do that though.