Exception when deserializing a file that I have previously serialized using System.Text.Json

In my C# (v9) .Net 6 app, I have a class with the following member variable:

private readonly List<Dictionary<ConsoleColor, ConsoleColor>> colours = new ()
{
    new Dictionary <ConsoleColor, ConsoleColor> (),
    new Dictionary <ConsoleColor, ConsoleColor> ()
};

I write it to json, in a file like this:

using System.Text.Json;
using System.Text.Json.Serialization;    

var options = new JsonSerializerOptions {
    WriteIndented = true,
    Converters = {
        new JsonStringEnumConverter (JsonNamingPolicy.CamelCase)
    }
};
string json = JsonSerializer.Serialize (this.colours, options);
using StreamWriter stream = File.CreateText (filePath);
stream.Write (json);

but when I try to read this back again as follows:

string json = File.ReadAllText (filePath);
var options = new JsonSerializerOptions
{
    PropertyNameCaseInsensitive = true
};
this.colours = JsonSerializer.Deserialize<List<Dictionary<ConsoleColor,ConsoleColor>>> (json,options);

I get an exception:

"The JSON value could not be converted to System.Collections.Generic.Dictionary`2[System.ConsoleColor,System.ConsoleColor]. Path: $[0].blue | LineNumber: 2 | BytePositionInLine: 18."

What am I doing wrong? The json looks good as a string - the exception occurs on the deserialisation (JsonSerializer.Deserialize). I think it might be expecting a number for the colour, but reading text instead, but I really do want my json to be human-readable (and in a pinch, editable).

The json file looks like this:

[
  {
    "blue": "gray",
    "cyan": "gray",
    "darkBlue": "darkGray",
    "darkCyan": "darkGray",
    "darkGreen": "darkGray",
    "darkMagenta": "darkGray",
    "darkRed": "darkGray",
    "darkYellow": "darkGray",
    "green": "gray",
    "magenta": "gray",
    "red": "gray",
    "yellow": "gray"
  },
  {
    "black": "black",
    "blue": "gray",
    "cyan": "gray",
    "darkBlue": "darkGray",
    "darkCyan": "darkGray",
    "darkGreen": "darkGray",
    "darkMagenta": "darkGray",
    "darkRed": "darkGray",
    "darkYellow": "darkGray",
    "green": "gray",
    "magenta": "gray",
    "red": "gray",
    "yellow": "gray"
  }
]