With the white ones, you can drop behind the scenes to escape enemies or find secret exits by holding down on the d - pad for 5 seconds. (gamerankings.com)
Take this pipe to reach the red flag secret exit. (lifewire.com)
The levels containing secret exits also disappear from the game entirely about midway through, leaving the first half of the game to feel more inspired than the second. (wizarddojo.com)