A better look at comparing the two (UIColor on the left, Color on the right), definitely a gradient still for both, but UIColor's redColor is darker than SwiftUI's

The gradient is still being applied when you do imageView.tintColor = .red, it's just that the UIColor.red and Color.red aren't the same colors so it looks different and is harder to tell. When you use UIColor(Color.red) as the tint it still works as seen here: