Color BackgroundColor { get; set; }

<DxChart Data="@WeatherForecasts"
         CustomizeSeriesPoint="@PreparePointLabel">
    <DxChartLineSeries SummaryMethod="@(i => (int)i.Average())"
                       ValueField="@((WeatherForecast i) => i.TemperatureF)"
                       ArgumentField="@(i => i.Date.Date)"
                       Name="Temperature, F">
        <DxChartSeriesLabel Position="RelativePosition.Outside">
            <DxChartSeriesLabelConnector Visible="true" Width="3" />
        </DxChartSeriesLabel>
    </DxChartLineSeries>
    <DxChartLegend Position="RelativePosition.Outside" />
</DxChart>

@code {
    WeatherForecast[] WeatherForecasts;
    // ...
    protected void PreparePointLabel(ChartSeriesPointCustomizationSettings pointSettings) {
        double value = (double)pointSettings.Point.Value;
        if(value > 25 && value < 75) {
            pointSettings.PointLabel.Visible = true;
            pointSettings.PointLabel.BackgroundColor = System.Drawing.Color.Red;
        }
    }
}