I do not see them in the screenshot. Because of that, I believe this is more than likely dead or stuck pixels on the LCD. Contact Dell if you are under warranty and have your LCD replaced. If you are not under warranty and want to resolve the issue you can find an LCD from either Amazon.com or partspeople.com
Open the LCD Repair website (LCD repair v2.0).
*Click “Fix My Screen.” Your browser will display a full-screen pattern of rapidly-flashing primary colors. Leave this pattern running on your screen for 10 to15 minutes.
*Press “Esc” to close the pattern and return to LCD Repair's menu screen. Check to see whether or not the dead pixel is now working again.
Or simply check Dell Latitude E6420 Manual to know what DELL experts advise you to fix this issue.