Why Preview Your Thumbnail?
Your thumbnail never exists in a vacuum. It competes with dozens of other images, colors, and texts on the screen. A design that looks great in Photoshop might vanish against YouTube's dark background or become unreadable when shrunk down to mobile size.
The "Context Gap": Most creators fail because they don't check legibility. By using the 4 modes above, you can ensure your text is large enough for mobile users (70% of traffic) and that your colors pop against the "Up Next" sidebar.