a yellow streak
A tendency to be cowardly or easily frightened. She likes to strut around like she's the boss of the place, but she has a yellow streak running down her spine if you ever confront her on an issue. He knew what they were doing was wrong, but his yellow streak kept him from intervening.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a ˈyellow streak(disapproving) the quality of being easily frightened; cowardice: He won’t fight? I always thought he had a yellow streak in him.
Yellow is often used to describe somebody who is a coward.
Farlex Partner Idioms Dictionary © Farlex 2017