Use an adjective after such verbs as be, become, seem, taste, smell, appear, sound, and look

From Ways of Thinking and Writing (1936) by Frank W. Cushwa and Robert N. Cunningham:

Use an adjective after such verbs as be, become, seem, taste, smell, appear, sound, and look unless you wish to qualify the verb. The adjective after the verb describes the subject.

Examples:

• He appears sick. [Sick describes the subject.]
• The rose smells sweet [not sweetly since a rose is incapable of the act of smelling].
• Though the entire team protested vehemently, the referee stood firm. [Stood firmly would describe the referee's posture; stood firm indicates that he was firm in his determination to enforce his decision.]