[dart] Arguments of a constant creation must be constant express

when developing app with flutter, I want to define some commonly used styles

the code is as follows:

import "package:flutter/material.dart";
class AppStyle {
  static Color colorRed = const Color(0xffe04f5f);
  static Color colorWhite = const Color(0xffffffff);
  static Color colorGreen = const Color(0xff1abc9c);
}

now, to define the style of a list title, add a line

  static TextStyle listRowTitle = const TextStyle(fontSize: 20.0, color: colorGreen);

if you write this above, there will be a problem here in colorGreen. The error message is

[dart] Invalid constant value.
[dart] Arguments of a constant creation must be constant expressions.
Color colorGreen

if, there is no problem changing colorGreen to Color (0xff1abc9c)!

static TextStyle listRowTitle = const TextStyle(fontSize: 20.0, color: Color(0xff1abc9c));

I have been google for a long time, but I still can"t find the answer. Which god can explain it to me?