Answer:
Sign stimulus
Step-by-step explanation:
Sign stimulus is a stimulus that will always produce the same behavioral response of an organism. In this case, red coloring is a sign stimulus for the fish attack which is its behavioral response.
This type of innate behavior is called Fixed Action Pattern (FAP) because it is always the same (there is usually no variants in the behavioral response actions). If the model is colored differently, there is no behavioral response (attack).