Given this C# code:

private static void SetColorInDictionary(ResourceDictionary resources, SolidColorBrush value)
{
    if (resources.Contains("TestDynamicResourceColor"))
    {
        resources["TestDynamicResourceColor"] = value;
    }
    foreach (var merged in resources.MergedDictionaries)
    {
        SetColorInDictionary(merged, value);
    }
}

var resources = Noesis.GUI.GetApplicationResources();
SetColorInDictionary(resources, new SolidColorBrush(Color.FromArgb(255, 255, 0, 0)));

DynamicResources do not revert back to their original values when restarting a game in the Unity editor.