WhatsApp do not allow to use external fonts in the messages. You only can use:
To type text in bold, place an asterisk before and after the text.
To write text in italics, place an underscore before and after the text.
To write strikethrough text, place a check mark before and after the text.
~ text ~
To type text in monospace, place three inverted quotes before and after the text.
That is all. Those are all the capabilities WhatsApp allows to use in terms of fonts.
You can not load new fonts or colors…
What you can do if you want to have more design, is to use status and create a new status with the design tool they have.