Undergrowth
Instant
As an additional cost to cast this spell, you may pay 
.
Prevent all combat damage that would be dealt this turn. If this spell's additional cost was paid, this effect doesn't affect combat damage that would be dealt by red creatures.
"The art of war is turning obstacle to advantage." —Jaeuhl Carthalion, Juniper Order Advocate
Illustrated by
Alliances#102bCommonEnglishLegality
Rulings
Oct 4, 2004
It means that red creatures still deal damage if you pay the additional cost.