Well the best way would be to buy a new monitor, because a monitor that came with a 486 is bound to be bad by now.

But anyways, does the monitor have any controls? What about a degauss\reset button on the back?

Sometimes a monitor plug inserted badly can cause color weirdness. Make sure it is in the computer completely.

And sometimes the display vibrating can be caused by electrical interference. Are there any magnetic devices near it? What about machinery? Sometimes even an intensive device on the same electrical circuit can cause problems (example: my computer is on the same circuit as the built-in vacuum, so when someone is cleaning the house, the monitor flickers)