From Wiki:
Black Decay
Time to break with diamond pick: aprox. 1.5 seconds.
Speed: Low.
Unique feature: Causes ground to collapse.
Possibly the most benign form of Decay, especially to Skylands or Void worlds. It mostly spreads to existing blocks, and it spreads faster up and down than it does to the sides. It also has the unique trait when two blocks form on top of each other, they will merge into one block of decay and cause every block above it to fall, even blocks that are normally unaffected by gravity. It is recommended to have a
Jetpack or other flying accessory to escape the decay, and floating structures unconnected to the ground are recommended. It can be blown up with TNT.
Blue Decay
Time to break with diamond pick: aprox. 2 seconds.
Speed: Medium.
Unique feature: None.
Blue decay travels moderately fast in all directions. It can travel through air, but moves around 5X slower than through existing blocks. It can be blown up with TNT.
Red Decay
Time to break with diamond pick: aprox. 3 seconds.
Speed: Medium.
Unique feature: None.
Similar to blue decay, but harder to clean up. It can be blown up with TNT.
Purple Decay
Time to break with diamond pick: aprox. 10 seconds.
Speed: Medium.
Unique feature: Spreads through air just as fast as the ground.
It breaks around as fast as obsidian, and behaves similarly to Blue and Red Decay, but without the speed penalty while traveling through air. It's nothing special if you are on the ground, but if you are in the air, it's even worse than the feared White Decay. It has an 85% chance to spawn in a Dense Ores World. It can NOT be blown up with TNT.
White Decay
Time to break with diamond pick: aprox. 10 seconds.
Speed: fast.
Unique feature: Can damage player.
White Decay is a death sentence for an Age. It acts like red and blue decay, but eats through the ground much quicker, it also eats through other forms of decay as other blocks, and slowly grows into the air. Possibly the only tool that can defeat White Decay is a
Vajra, and then only if the decay has not gone far. It is also the only decay that can physically harm the player. It deals an armor-penetrating half-heart of damage per second, but only if the player is moving. It can NOT be blown up with TNT.