One of Trunks' strengths is his focus, and while he's aware he's bright red! There are people watching! Most of his focus is taken by Cassie, and kissing her. Matching her.

She may be grinning from ear-to-ear, but he looks a little dazed, then ducks his head in embarrassment. Especially since he can hear the whispers, and feel the pressure of the staring.