Make UIColor Codable

If you care only about the 4 color components this is a simple solution using a wrapper struct

struct Color : Codable {
    var red : CGFloat = 0.0, green: CGFloat = 0.0, blue: CGFloat = 0.0, alpha: CGFloat = 0.0
    
    var uiColor : UIColor {
        return UIColor(red: red, green: green, blue: blue, alpha: alpha)
    }
    
    init(uiColor : UIColor) {
        uiColor.getRed(&red, green: &green, blue: &blue, alpha: &alpha)
    }
}

In this case you have to write a custom initializer to convert the 4 color components from Color to UIColor and vice versa.

struct MyTask: Codable { // renamed as MyTask to avoid interference with Swift Concurrency
    
    private enum CodingKeys: String, CodingKey { case content, deadline, color }
    
    var content: String
    var deadline: Date
    var color : UIColor
    
    init(content: String, deadline: Date, color : UIColor) {
        self.content = content
        self.deadline = deadline
        self.color = color
    }
    
   init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        content = try container.decode(String.self, forKey: .content)
        deadline = try container.decode(Date.self, forKey: .deadline)
        color = try container.decode(Color.self, forKey: .color).uiColor
    }
    
    public func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(content, forKey: .content)
        try container.encode(deadline, forKey: .deadline)
        try container.encode(Color(uiColor: color), forKey: .color)
    }
}

Now you can encode and decode UIColor

let task = MyTask(content: "Foo", deadline: Date(), color: .orange)
do {
    let data = try JSONEncoder().encode(task)
    print(String(data: data, encoding: .utf8)!)
    let newTask = try JSONDecoder().decode(MyTask.self, from: data)
    print(newTask)
} catch {  print(error) }

A smart alternative for Swift 5.1 and higher is a property wrapper

@propertyWrapper
struct CodableColor {
    var wrappedValue: UIColor
}

extension CodableColor: Codable {
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let data = try container.decode(Data.self)
        guard let color = try NSKeyedUnarchiver.unarchivedObject(ofClass: UIColor.self, from: data) else {
            throw DecodingError.dataCorruptedError(
                in: container,
                debugDescription: "Invalid color"
            )
        }
        wrappedValue = color
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        let data = try NSKeyedArchiver.archivedData(withRootObject: wrappedValue, requiringSecureCoding: true)
        try container.encode(data)
    }
}

and mark the property with @CodableColor

struct MyTask: Codable {
    var content: String
    var deadline: Date
    @CodableColor var color: UIColor
...
}