Color property type

The color property type accepts string values with the format of colors. It provides a dialog for picking a color and returns it with a hex format (including the alpha component).

The color property editor.

The scene compiler generates the code for defining a Color property like this:

class GoToSceneButton extends Phaser.GameObjects.Image {

    ...

    /** @type {string} */
    fadeOutColor = "#ffffffff";
}

And generates the code for setting the property in a scene:

// goToSceneButton (prefab fields)
goToSceneButton.fadeOutColor = "#ff00b5ff";

There are many different ways of representing a color, you can use the Phaser.Display.Color class for converting to different formats.