Quick test case for inconsistent Xamarin Forms frame background color.
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In Xamarin Forms on iOS, the frame control background color does not match the color of other elements due to differences between CGColor and UIColor.

Most iOS renderers work using UIKit and UIColor. The background color for instance is set using UIView.BackgroundColor.

Frame however works using CoreGraphics and CGColor and manipulating a view's layer. The background color is set using UIView.Layer.BackgroundColor.

For cross platform support, Xamarin Forms defines it's own Color class and has helper methods to convert to the platform colors. The iOS conversion to CGColor here is a little naive.

Both the ToCGColor and ToUIColor methods simply take the red, green, blue, and alpha components of the Xamarin Color directly. iOS is a bit more sophisticated than that, in that it supports different color spaces, making the conversion between CGColor and UIColor more complicated.

The bottom line, for consistency, ToCGColor should be defined as follows:

public static CGColor ToCGColor(this Color color)
  return color.ToUIColor().CGColor;

Below is a screenshot with a frame inside a contentview, with the same Xamarin Color as the background color for each.