Elemental Wand/Staves

I remember reading about this ages ago, but I've completely forgotten about how it works.

So elemental wands/staves give bonuses for elements that match the colour right? how much does it increase those by? (I remember that it was lowered at one point)

I also heard of a damage reduction if you use the wrong element. Is this true? how much is the reduction?

Could I use an Ice/Lightning Elemental Wand on my Bishop without a reduction?

And are Battle Mages affected by this reduction (I doubt this)

thanks!