You might want to upload a new image with white background as your blogger header image. Unfortunately, in most cases, the white color turns to grey in the blog. A bug in blogger converts the white background into grey. This color change will not be very pleasing to the eyes too.
How To Convert Blogger Header Image From Grey To White?
The issue is because of Google's auto enhance feature in Google plus. To fix this issue, go to your Google Plus Settings and scroll down to Photos> Auto Enhance. Now uncheck 'Automatically Enhance New Photos' and save the settings. Now your blogger header image will show the color of the original image.You can read my 'Never Revealed Blogger Tips' I learned through my years of experience in blogging.