That's not much to go on. Here's a guess -- you added the Fill effect to a layer, and Fill defaults to red. The fix: delete the effect.
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And don't EVER try to use AE intuitively. One of two things is bound to happen -- you'll get hopelessly lost or you'll fail. Neither is good. Instead, learn the Basics of AE. Walk before you run. And learn from a PRO and not some high school kid on YouTube:
You'll thank us later.