No color - black and white picture

This means absolutely no color - equivalent to a black and white picture.
Not even a hint of color.

If you are using a composite video input, troubleshoot the chroma circuitry
like you would a TV - see the document: "Notes on the Troubleshooting and Repair of Television

This is an extremely unlikely failure mode for a computer monitor
unless you are using a composite video input. It is most likely to
a software driver or program problem. Sometimes, the PC will think that
the monitor you have connected is not capable of color and certain programs
will then display in B/W no matter what.

In some cases this is due to an initialization problem - possibly a race
condition during the boot process - especially likely if you are using an
older video card with a new fast processor.

First, confirm that the source is actually in color - try the monitor on
another computer or vice-versa.
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Check the settings of any mode switches - in rare cases there is a color/mono
switch or button.

Note that to the average person, the obvious question becomes: is my color
picture tube bad? The answer is a definitive NO. It is virtually impossible
for a defective CRT to cause a total loss of color. A defective CRT can
cause a lack of a primary color - R, G, or, B which will mess up the color
but is not likely to result in a black and white picture.