How do I program a "flash" effect when updating text boxes in a windows form with C#?

You could spin off a seperate thread per flashing textbox as to not block your form from being used during the flashing of your textbox(s). Be sure to invoke your form as spinning of the thread will require cross threading. Full solution below.

private void Form1_Load(object sender, EventArgs e)
{
    // textBox1 is the control on your form.
    // 1000 is the total interval between flashes
    // Color.LightBlue is the flash color
    // 10 is the number of flashes before the thread quits.
    Flash(textBox1, 1000,Color.LightBlue,10);
    Flash(textBox2, 1500,Color.Green,10);
    Flash(textBox3, 100,Color.Red,10);
    Flash(textBox4, 500,Color.Brown,10);
    Flash(textBox5, 200,Color.Pink,10);
}

public void Flash(TextBox textBox, int interval, Color color, int flashes)
{
    new Thread(() => FlashInternal(textBox, interval, color, flashes)).Start();
}

private delegate void UpdateTextboxDelegate(TextBox textBox, Color originalColor);
public void UpdateTextbox(TextBox textBox, Color color)
{
    if (textBox.InvokeRequired)
    {
        this.Invoke(new UpdateTextboxDelegate(UpdateTextbox), new object[] { textBox, color });
    }
    textBox.BackColor = color;
}

private void FlashInternal(TextBox textBox, int interval, Color flashColor, int flashes)
{
    Color original = textBox.BackColor;
    for (int i = 0; i < flashes; i++)
    {

        UpdateTextbox(textBox, flashColor);
        Thread.Sleep(interval/2);
        UpdateTextbox(textBox, original);
        Thread.Sleep(interval/2);
    }
}

This avoids having to put supporting timer controls on your form.

The WPF would seem perfect for this. You can build it in WPF and use it in WinForms as a HostedElement. Add new project WPF User Control, and this in xaml:

<UserControl x:Class="WpfControlLibrary1.UserControl1"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         mc:Ignorable="d" 
         d:DesignHeight="20" d:DesignWidth="300">
<TextBox>
    <TextBox.Triggers>
        <EventTrigger RoutedEvent="TextBox.TextChanged">
            <BeginStoryboard>
                <Storyboard AutoReverse="False" BeginTime="0" >
                    <DoubleAnimation Storyboard.TargetName="Foo"
                                 Storyboard.TargetProperty="Opacity"
                                 From="0" To="1" Duration="0:0:1"/>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
    </TextBox.Triggers>
    <TextBox.Background>
        <SolidColorBrush Opacity="1" x:Name="Foo" Color="LightGray" />
    </TextBox.Background>
</TextBox>
</UserControl>

(it could do a little work, but it's a start). There you have it - a fancy textbox :)

Build the solution, and a new item will appear in Toolbox - just drag & drop to your Form, the WPF will be hosted inside ElementHost element. The beauty of it is that you can do much much more in visual styles in WPF, however, it's hosted WPF which adds certain weight to your solution...

Derive your own class from TextBox. Give it a Flash() method that starts the flashing. Just change the BackColor to a pastel color. Don't use alpha, that doesn't work on a TextBox.

You ought to have all instances of this class share a common Timer so they will flash at the same time. Make the timer static and reference-count the number of instances you have. Add up in the constructor, down in the Dispose(bool) override.