It's a smart question. It wouldn't seem to make much difference since you'd assume people would have channels they're subscribed to or search for something spec

It's a smart question. It wouldn't seem to make much difference since you'd assume people would have channels they're subscribed to or search for something specific. Much of YouTube watch time is based on discovering new videos. And a big part of that is the thumbnail people see when they're looking at suggested content, related content, or search results. Colors, whether there are faces, fonts, images, etc all make a difference. And it varies over time and genre. YouTube has tools built in so you can A/B test different thumbnails and automatically select the better performing one. Note that I haven't done any of this myself except for making a couple thumbnails for personal videos. I was curious and watched a YouTube video about thumbnails and why they're important.