So, I read this explanation on Reddit and decided to bring it here. Here's a quick resume:
• #5's story isn't a flashback, as it doesn't have a voiceover narration. The "grey tone" is to match Hester's story from earlier, and it's not being told.
• After Boone kills Earl, it cuts to #5 driving, then the Red Devil kills the truck driver and #5 runs back to Kappa, where she tells the events in a resumed version to her sisters and Chanel criticizes her.
+ So if #5 is telling someone the story as we see the attack scene, there's no evidence to see who she's speaking to, plus no voiceover to indicate it's a story.
Because people are saying that this story is an indication as to why #5 is the killer, and it may not actually be an indication :d