@msbellows @Miriamm "be" is one form of the verb "to be", but it's only an infinitive when a "to" prepends it. An infinitive in English is a verb inflected by "to:" "to buy," "to be," "to speak."

There are plenty of infinitives that have nothing to do with "be," e.g. "I want Steve to buy coffee." "to buy" is an infinitive in that sentence.

"is purple" is not an infinitive structure. "to be purple" would be an infinitive structure.