Essentials Of Swedish Grammar -

Röda bilar (red cars)—the adjective typically ends in -a .

Adjectives must agree with the noun they modify in both gender and number. En röd bil (a red car). Essentials of Swedish Grammar

For more in-depth study, the Essentials of Swedish Grammar by Åke Viberg is a widely recommended practical guide. Röda bilar (red cars)—the adjective typically ends in -a