A bright thumbnail is not automatically a clear thumbnail. Color works best when it helps the important subject stand apart and supports the video's tone.
Start with the existing image
Look at the dominant colors in the subject and background. If they are nearly identical, first consider a different crop or image. Trying to rescue poor separation with many effects can make the result look artificial without improving recognition.
Use a limited palette
Choose a small set of colors with distinct jobs: subject, background, and any necessary text or accent. Keep the main subject visually dominant. A collection of equally intense colors gives the viewer no clear indication of where to look.
Check brightness as well as hue
Two different colors can still have similar lightness and become difficult to distinguish. View a grayscale preview as a practical check of separation. If an important label or edge disappears, improve the contrast rather than assuming that different hues are enough.
Match the promise and mood
A calm instructional video does not need the same visual treatment as a dramatic fictional scene. Color should support the expectation the upload creates. Review the thumbnail beside several earlier uploads to see whether it remains recognizable without forcing every subject into an identical color formula. Clarity and truthful presentation matter more than making every pixel compete for attention.