I use SCNScene.write To file ".usdz", then open the ".usdz" by SCNScene.url , all Nodes color fades.
Export and load only once. The fading is not obvious. Return and repeat 4 or 5 times. It is obvious that the color is inconsistent with the original color and has become much lighter.
Our engineering teams need to investigate this issue, as resolution may involve changes to Apple's software. Please file a bug report, include a small Xcode project and some directions that can be used to reproduce the problem, and post the Feedback number here once you do. If you post the Feedback number here I'll check the status next time I do a sweep of forums posts where I've suggested bug reports.
Bug Reporting: How and Why? has tips on creating your bug report.