using System;

using System.Collections.Generic;

using System.Runtime.Serialization;

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

​

public class Program

{

    public static void Main()

    {

        var json = @"

        {

            ""2021-11-29"":{

              ""color"": ""red""

           },

           ""2021-12-19"":{

              ""color"": ""blue""

           },

           ""2021-12-20"":{

              ""color"": ""green""

           }

        }";

        var deserialized = JsonConvert.DeserializeObject<Root>(json);

        foreach (var record in deserialized.Records) 

            Console.WriteLine($"{record.Key} = {record.Color}");

    }

    public class Root { 

        [JsonExtensionData]

        private Dictionary<string, JToken> _data { get; set; } 

        public List<Preference> Records { get; set; } = new List<Preference>(); 

        [OnDeserialized]

        private void OnDeserialized(StreamingContext context)

        {

            foreach (var key in _data.Keys)

            {

                var data = _data[key].ToObject<Preference>();

                data.Key = DateTime.Parse(key);

                Records.Add(data);

            }

        }

    }

    public class Preference 

    {

        public DateTime Key { get; set; } 

        public string Color { get; set; }

    }

}