User blog comment:Zolnir/Dust, The Ultimate Theory (Refined)/@comment-23784953-20130815123847/@comment-11188061-20130815134220

There's a much simpler explanation for Weiss' Red mode. One, her Red Dust is extremely pure and thus contains a lot of energy. Second, the Red Dust is the most closely related with strength/power. Look at it this way:

Red: Fire, Strength, Yellow: Thunder, Speed Blue: Ice, Defense

It's simple and easy to understand, which fits the anime medium. When Weiss slammed her rapier against the knight's, the sheer power contained in Red repelled the sword like it's made out of candy.

Yellow should be also be associated with ignition/reaction. This is one of the major reasons why combining Fire and Thunder (Orange) dust creates a massive explosion.

As for the Roman Torchwick situation, It's not as complicated as you think. It'll be easier to assume that he fired out a simple Dust energy, whatever the element is (it might even be no element, plain grey Dust. Remember Ruby's and Yang's weapons).The Dust energy slammed into the crystal, caused a reaction and thus the Fire Dust exploded.