Cheapest priced copy (Alliances): $0.35 at Card Kingdom, near mint, their price list as of .
Undergrowth 
Instant · Common · Alliances · #102a
As an additional cost to cast this spell, you may pay {2}{R}.
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.
Legal inCommanderLegacyVintagePauper
Not legal inBrawlStandardPioneerModern
2 printings · $0.35 at Card Kingdom
One ruling from Wizards of the Coast
- 4 October 2004It means that red creatures still deal damage if you pay the additional cost.