The veins on our white marble shower are turning yellow. How can we remove the discoloration?

There are two possible reasons the veins in your marble are changing color. First, white marble characteristically contains iron; it is possible the iron is oxidizing and turning to rust. If this is the case, it may be impossible to remove. Second, have your water checked for iron. If your water contains iron it may be being deposited in the veins of the marble. If this is the case, it may be possible to remove the iron using some of the commercially available iron removers. But if you decide to use one of these formulas, proceed with caution; many contain strong acids which can damage the marble.