Hi Leon

Thanks for your suggestion. I have tried it, but it didnt work.

I am on higher version of Xamarin.Forms then 5.0.0.2196. My version is 5.0.0.2401

Style="{StaticResource MButton}" in Appl.xaml is as follows:

    <Style x:Key="MButton" TargetType="Button">
                        <!--<Setter Property="CharacterSpacing" Value="4" />-->
                        <Setter Property="TextColor" Value="AntiqueWhite" />
                        <Setter Property="BorderRadius" Value="3" />
                        <Setter Property="FontSize" Value="8" />
                        <Setter Property="FontAttributes" Value="Bold"/>
                        <Setter Property="TextTransform" Value="None"/>
                        <Setter Property="BackgroundColor" Value="DarkCyan"/>
                    </Style>

I have applied your code to binding of MButtBckColor1:

    Xamarin.Forms.Color mButtBckColor1 = Xamarin.Forms.Color.White;
            public Xamarin.Forms.Color MButtBckColor1
            {
                get { return mButtBckColor1; }
                set
                {
                    mButtBckColor1 = value ; 
                    OnPropertyChanged(nameof(mButtBckColor1)); 
                }
            }


ViewModel code first time round is 

                        MText1 = "Color Test";
                        MButtBckColor1 = Xamarin.Forms.Color.DarkCyan;

The way I want App to works is:

First screen displays 16 squares (Buttons) with DarkCyan as Background Color:

User selects squares and color changes. Eg Sq 1 changes to gray:

Then they press a button:
<Button Text="Next" Grid.Row="1" Grid.Column="1"  BackgroundColor="Green" TextColor="Black" TextTransform="None" Command="{Binding ActionMCommand}"></Button>

ActionCommand is in ViewModel:

 public ICommand ActionMCommand 
        {
            get
            {
                return new Command(() =>
                {
                    SelectMData();
                    ResetCellsData();
                });
            }

        }

ResetCellsData resets the button color to default:

        public void ResetCellsData()
        {
            MButtBckColor1 = Xamarin.Forms.Color.DarkCyan;
		\\ other Buttons
	  }

ResetCellsData gets called and sets the 
		MButtBckColor1	R=0, G=0.545098066329956, B=0.545098066329956, A=1, Hue=0.5, Saturation=1, Luminosity=0.272549033164978	Xamarin.Forms.Color

But the MButtBckColor1 does not change when it gets refreshed. It stays Grey.