WPF Binding brush generates marshalling error

I'm quite confused as at runtime I get this error: The application called an interface that was marshalled for a different thread. (0x8001010E (RPC_E_WRONG_THREAD))'

I have a on object that has:

            <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto" />
                <ColumnDefinition Width="*" />
            </Grid.ColumnDefinitions>
            <TextBlock
                Grid.Column="0"
                FontFamily="Segoe MDL2 Assets"
                Foreground="{Binding ImageColor, Mode=OneWay}"
                Text="{Binding ImageSrc, Mode=OneWay}" />
            <TextBlock
                Grid.Column="1"
                VerticalAlignment="Center"
                Text="{Binding VisitDescr, Mode=OneWay}" />
        </Grid>

in the class I have

    public string ImageSrc
    {
        get
        {
            switch (Type)
            {
                case "c1":
                    return "\xE8B9";
                    break;
                default:
                    return "\xE8A5";
                    break;
            }
        }
    }

    public Brush ImageColor
    {
        get
        {
            switch (Type)
            {
                case "c1":
                    return new SolidColorBrush(Colors.MediumOrchid);
                    break;
                case "c2":
                    return new SolidColorBrush(Colors.DarkOrchid);
                default:
                    return new SolidColorBrush(Colors.Red);
                    break;
            }
        }
    }

The image is returned without any error but the Color generate the error and is NOT changed.

How is it possible I get a marshalled error?

I've found the issue... it was using the work library. I've updated it to:

using System.Windows.Media;

and now is working. But why it got Marshalling and not type conversion error?