Default advancement description color depends on frame, rather than being described in the JSON text-component

The bug

In this pre-release the descriptions of advancements have gotten colors based on their frame (challenge = dark_purple, frame and task = green). These colors should be described in the text-component of the advancement's description, rather than default to a certain color based on the frame.

The fix

Make the descriptions of advancements have a "color" key which sets the color, rather than the frame setting the default color.