When reading this question, I noticed something on the screenshot of the accepted answer that I never saw in my own game despite collecting 100% plants: silver-colored pumpkins.
What are they and where do I get them?
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This "silver pumpkin" is simply an Imitater, used to bring an additional pumpkin card to the level.
When you select the imitater as one of your plants for the level, you then need to select which plant it will mimic. For that level, placing the imitater on the grass will have it appear briefly, before turning into a grayscale (well, gray-ish) version of the plant you chose for it to imitate.