As far as I know(which is about... 3 metres

) you use your sword for offensive attacks, never defensive. Thats why they created the Shield
If I get this right, you refer to the sparks (or something) that appears when you hit an enemy(red sparks/flash/whatever) or when an enemy hit your shield(blue sparks/flash/whatever).
I guess the main reason is varioation. And naturally, offense is red and defense blue, anything else would look silly(maybe red and green would work but why change the classics, everybody gets it)