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.
"It is the work of sorcery. Burn it!" —Taaveti of Kelsinko, Elvish Hunter
Illustrated by
Alliances#102aCommonEnglishLegality
Rulings
Oct 4, 2004
It means that red creatures still deal damage if you pay the additional cost.