That’s because the digital paperback cover is in the RGB color space while your physical book cover was printed in CMYK. (Amazon KDP converts the RGB file into the CMYK format at the printer.)
RGB is used for digital images and looks brighter/more vivid. CMYK is used for printing, and it’s difficult for the printer to match the digital color quality when using ink or toner on paper. CMYK prints dark. For most covers, there will be a slight difference of color from the digital cover on your screen and the book you hold in your hands.
However, if your paperback cover has lots of dark colors and blackness in it, I will need to increase the brightness and contrast, otherwise the printed copy will be way too dark and muddied. What I’ll do is make sure the colors in the paperback cover matches the ebook color scheme and send you the .jpg for your records and promotion purposes. Then I’ll adjust the colors and make it brighter. I’ll send you this altered cover in print ready .pdf for you to upload to KDP. Don’t worry. It won’t print being so shiny and bright looking. Since CMYK prints dark, it will reduce the brightness of your .pdf. The paperback cover (what you hold in your hands) will be similar in color to the digital version on your screen.