If you want a true to style Hefe Weizen then you have to use a Hefe Weizen/Bavarian Weizen yeast.

Anything else is NOT a German-style HW.
For the banana flavor you need to ferment at or above 70F. That does not mean you can't get similar flavor at lower temps, but it does depend on the yeast. Read the yeasts profile to determine if what it does is what you want in the final product.
As for the color getting lighter? I've never heard (of it) or experienced that before.
