Check the cables to make sure they are connected properly and/or that they aren't damaged.
If you have access to another machine/monitor, test the monitor on the other machine or a known working monitor on your machine.
If the issue happens on another machine, time for a new monitor soon.
If another monitor has the same issue on your machine, you may need to update the video drivers or purchase a new video adapter.

I have also had this problem but not only on my computer which was on windows
98, has recently been updated with windows xp hoped this may sort problem,
but on another family member's computer which was relatively new (1 year or
so) again watching movies etc the screen just turns to a bottle green for
varying lengths of time and can just as suddenly turn back to blue and a lot
brighter. That computer is a different model, but also uses windows xp, I
have tried installing any updates, clearing unwanted files, as I also get a
message saying not enough vitual memory, my computer and monitor is just
under 10 years old, are there any other solutions than trying new monitors or
video cards( sounds expensive) could it be there isn't enough processing
power or memory? don't know much about computers but nay help would be
appreciated!