It is possible there are more than 1
images of the Cover
Notice my wording?
The image Marked as cover may be Greyscale. That may not be the image used at the beginning of the book (that you think of as
cover )
Unpack (Tap U) the AZW3 , what files do you see (may be in Images folder)

In Windows you can rightclick in the file list and change the view to Thumbnails or Extra Large... no need to open)