Funciona para mim.
Add-Type-AssemblyNameSystem.Windows.Forms$Form=New-Objectsystem.Windows.Forms.Form$Form.Width=300$Form.Height=200$StatisticsBox=New-ObjectSystem.Windows.Forms.RichTextBox$StatisticsBox.Location=New-ObjectSystem.Drawing.Size(50,30)$StatisticsBox.Size=New-ObjectSystem.Drawing.Size(150,100)$StatisticsBox.MultiLine=$True$StatisticsBox.ScrollBars="Vertical"
$StatisticsBox.ReadOnly=$True
$Form.Controls.Add($StatisticsBox)
$StatisticsBox.SelectionColor = 'red'
$StatisticsBox.text = "'r'nNot Found:'r'n" + $StatisticsBox.AppendText("colored stirng") + "'r'n" +$StatisticsBox.text
[void]$Form.ShowDialog()
$Form.Dispose()
POSHGUI é sempre a minha primeira parada para isso.