public class MyGroupHeaderTextColorConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        // The BindingContext of your DataTemplate is GroupHeaderContext
        if (value is Telerik.XamarinForms.DataControls.ListView.GroupHeaderContext context)
        {
            var headerText = context.Key.ToString();

            switch (headerText)
            {
                case "Dairy":
                    return Color.Gold;
                case "Bakery":
                    return Color.SandyBrown;
                case "Meats":
                    return Color.IndianRed;
            }

            // other properties of the context that you can use

            // context.IsExpanded
            // context.Items
            // context.Level
        }

        return Color.Black;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}