LCD monitor have stuck or dead pixels? According to the tips in the article, “dead pixels appear black while stuck pixels can be one constant color like red, green or blue”.

See what you may be able to do to fix stuck pixels or whether the monitor manufacturer will replace the monitor if you have enough bad/dead pixels. Along with ideas for “massaging” the stuck pixel, there are also suggestions of using flashing video files which change colors to “unstick” the pixel(s):
How to Fix a Stuck Pixel on an LCD Monitor

Be sure to see the External Links section which includes a link to JScreenFix, a free Java applet that reportedly helps fix stuck pixels on LCD screens and removes burn-in / image persistence by randomly and rapidly switching on and off red, green and blue color components.

If a pixel is dead (shows up as black instead of colored), you’ll need to use a different fix. The solution explained here may help in that case:
Fix your LCD’s dead pixels