Hey there kindermilo,
Thank you for using the Apple Support Communities! I understand you are noticing a color difference with the selected wallpaper on your iPhone. I'd like to help.
Is this happening with all wallpapers selected? If you restart your iPhone, Restart your iPhone - Apple Support, does that help improve the colors? When you navigate to Settings > Accessibility > Display & Text Size > Color Filters, is this Off?
I would suggest checking under Settings > General > Software Update, and completing any available updates to see if this helps improve what you're seeing: Update your iPhone, iPad, or iPod touch - Apple Support.
Best regards.