That’s because the digital cover is in the RGB color space while the printed paperback cover is in CMYK. 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.
So please remember that the digital proof of your ebook and paperback wrap (whether it’s printed from Amazon KDP or IngramSpark) will be a little brighter than the actual paperback that you’ll hold in your hands.