Originally Posted by: Dave CardwellThanks for the help. :) Is the main purpose of using an altered dominant to have different colors in the chords?

That's one reason. The other main reason is to do with tension and release. If you play a dominant chord (or 5th chord eg. D7), it naturally wants to go to the 1 chord (Gminor or Gmajor). Altering the 5th and 9th adds more tension and makes the transition from 5 to 1 even more satisfying.

Compare:

Dm7b5 - Gm7 - Cm

Dm7b5 - G7 - Cm

Dm7b5 - G7#5b9 - Cm

When you play those you should notice how the G7 wants to go to the Cm more than the Gm7 does. when you change the G7 to a G7#5b9 it enhances that effect even more.

I hope that made sense :)