Button Use Rules:

Colors

Radical Red (pink) buttons are to be used only for a major call to action or final step in a process or leading to a payment.

Denim Blue (dark blue) buttons are the main button color and are to be used for any standard call to action or for a Next > button it a set of steps. The final step of that process may have a Denim Blue button or a Radical Red button. Refer to the visual spec for the color choice unless the final step is a payment. If the final step is a payment, the button will be Radical Red.

Cornflower Blue (light blue) buttons are used as secondary buttons on a page or for minor calls to action on a page. Refer to the visual spec for their use.

Silver (grey) buttons are used as Cancel or Back buttons. 

Arrows
Arrows are used on buttons when initiating the start of a flow or when taking a step within a flow. They can signify moving forward to getting started with a process.

Left/Back arrows are used when stepping back in a process.

Disabled State
Button transparency reduces to 50% in their disabled state.