RainbowGradientXamFormsButton.xaml

<Button Padding="2"
                    Text="Analyse Sentiment"
                    TextColor="Black">
                <Button.Background>
                    <LinearGradientBrush EndPoint="1,0">
                        <GradientStop Color="Red"
                                      Offset="0.1"/>
                        <GradientStop Color="Orange"
                                      Offset="0.225"/>
                        <GradientStop Color="Yellow"
                                      Offset="0.35"/>
                        <GradientStop Color="LightGreen"
                                      Offset="0.475"/>
                        <GradientStop Color="CornflowerBlue"
                                      Offset="0.6"/>
                        <GradientStop Color="BlueViolet"
                                      Offset="0.775"/>
                        <GradientStop Color="Purple"
                                      Offset="1.0"/>
                    </LinearGradientBrush>
                </Button.Background>
            </Button>