"The red flag!"
Translation:¡La bandera roja!
In most cases in Spanish, the adjective comes after the noun. There are some exceptions to this rule (e.g. "Él es un buen hombre", "He is a great man"), where the adjective is placed directly before the noun. I believe the adjective is in front of the noun in a few select cases where the adjective describes the noun in a more emotional and sentimental sense (can a native speaker confirm this for me?), but most often it follows the noun it describes. Therefore, "La bandera roja" is correct; not "La roja bandera".
With some of the Spanish vocabulary, such as the nouns "muchacho/muchacha", the ending can be changed to have essentially the same noun but a different gender. This, however, is not the case for most Spanish words. The Spanish word for "flag" is feminine: "la bandera". "el bandero" is not a Spanish word at all, to my understanding.