InteriorLightingWithProtocolExtension.swift

import UIKit

protocol InteriorLighting {
    var interiorLightColor: UIColor { get }
}

extension InteriorLighting {
    var interiorLightColor: UIColor {
        return .white
    }
}

struct Car: InteriorLighting {}

struct Truck: InteriorLighting {}

print(Car().interiorLightColor) // UIExtendedGrayColorSpace 1 1
print(Truck().interiorLightColor) // UIExtendedGrayColorSpace 1 1