Upon entering them, the Sonics can play a specific Challenge Act. Once that mission has been cleared, a golden bell, called a Bonus Bell, will appear on top of the Challenge Act Gate. When these bells are rung, they will release a Music Note that contains collectible music or artwork. These notes however, has to be caught within twenty seconds or they will fade away.
Just as the Sonics can restore color to a level by clearing all of the level's acts, the Sonics can restore more color to the White Space by clearing the Challenge Act Gates. For every fifth Challenge Act Gate the Sonics clear in an area, a part of the color in the White Space will be restored.
The Challenge Act Gate appearance varies for Classic and Modern Sonic. For Modern Sonic, the Challenge Act Gates will have the curve at the top and the flat end at the bottom, while for Classic Sonic, this order is reversed.
Type of Challenges Act Gates
Each Challenge Act Gate contains a mission of a certain type which is marked by an icon on top of them:
- Speed: The Sonics have to race through an obstacle course within a certain time limit.
- Technique: Serving as more technical missions, the Sonics have to utilize skills and tactics to pass the mission as fast as possible.
- Doppelganger: The Sonics have to race against a Sonic copy throughout the levels and reach the goal first.
- Friends: The Sonics have to team up with their friends to clear the mission or compete against them and win.
- Enemy: The enemies has been tweaked to make them more challenging (such as increasing their speed, number or size) and the Sonics have to pass through them and reach the goal.
- Item: The Sonics (mostly Classic Sonic) have to use Item boxes in order to clear the mission and reach the goal.
- Ring: The Sonics have to clear missions regarding Rings, such as to pass through an act with only one Ring or collect a set amount of Rings and reach the goal.
- Juggle: Classic Sonic has to bring the Goal Plate to the end of the level without dropping it.
- Rescue: Classic Sonic has to free a certain amount of Animals and reach the goal before running out of time.
- If an S rank is achieved on the mission, the gate will sparkle.