A boy looks out of a window at night and sees different kinds of colored monsters (green goblin, blue unicorn, or some such, a new one every page) in the tree outside his window but isn’t scared because they aren’t real. Finally he sees a brown owl and IS scared (because the owl is real). I had this book in the 1980s. The words are a repetitive pattern each page until he sees the owl.
Could be It Didn’t Frighten Me by Janet Goss. https://www.goodreads.com/book/show/2837840-it-didn-t-frighten-me
Here’s someone reading it on youtube https://www.youtube.com/watch?v=S3kUT0B07Eo
Yes!!! That’s it!!! Thank you so much!!