Reviewers often echo the preference implied by a question. Neutral prompts help you learn what the thumbnail communicates before explaining what you hoped it would communicate.
Avoid the preferred answer
Instead of asking whether the brighter version is more exciting, ask what each version suggests about the video. Show the drafts in a neutral arrangement and keep your own opinion private until the initial responses are recorded.
Ask for observations
Useful questions include which object is clearest, which words can be read, and what the viewer expects to learn. Follow up on uncertainty rather than asking for a numerical rating with no defined meaning.
Interpret the sample honestly
A few reviewers can reveal obvious confusion, but they do not represent every potential viewer. Record their familiarity with the subject when relevant. Use repeated misunderstandings to improve the design, and keep personal aesthetic preferences separate from factual errors or failures to communicate the video's task.