Hi,

In WordPress dashboard, click on the Contact menu and edit your form:
Your button is at the bottom of the form, it starts with [submit ...

To change the color of the button change class:btn-c1 to btn-c2 for secundary or btn-c3 for your tertiary color. Or btn-blue, btn-green, btn-pink, btn-orange, btn-black, btn-white for predefined colors.

If you want to match the styling of the buttons above the form, you can select btn-white. You can also add the wired class to it (as buttons above) in this case you would add class:btn-wire.

so the whole example would be:

[submit class:btn class:btn-white class:btn-lg class:btn-wire "Send"]

I hope this helps. Let me know if anything unclear. 

Kind regards,
Andrej