There are several verbs in English meaning "to cause to become [a given color]". Most of these, it seems, end in the suffix "-en". There are other adjectives as well that use "-en" to create causative verbs; "harden" and "fasten" come to mind.
The verb meaning "to make brown, tthough, is simply "brown". I can't think of any other verb of this class that uses the bare adjective form without a suffix. Does "brown" have a different history than other causative verbs formed from adjectives? Or why did it, and seemingly only it, retain the adjective form?