I recently found the tinted app icon feature, which I love I think it is such a cool idea. The execution on the other hand is slightly flawed, I tried to set my apps to the light pink color. The feature work well in the editing area but once I clicked done the apps went from a light pink to dark red. I have tried this multiple times with multiple colors and the issues are consistent through all colors except when using the color dropper.
IOS 18.0 Bug
It's a beta. Not every feature is going to work 100% correctly immediately.
Have you raised a Feedback report? If not, please do so at https://www.apple.com/feedback/ then post the FB number here.
Please file a bug report via Feedback Assistant. Include a screen recording that shows the transition from editing to finished icon.