Why does eyes go yellow when he is in his Hollow form?

The yellow eyes in Ichigo's Hollow form are a result of his heightened spiritual power and connection to his inner Hollow. When Ichigo first undergoes his Hollow transformation, his eyes turn yellow to symbolize the emergence of his inner Hollow and the increase in his spiritual power. It's a physical manifestation of his internal struggle between his human and Hollow sides. As Ichigo gains more control over his Hollow powers and strengthens his connection with his inner self, the yellow color of his eyes becomes more intense, reflecting his growing mastery over his Hollow form and its abilities.