The must common faults of a missing color a
1. A break in the color video signal wire within the video cable.
This usually occurs near the end that connects to the computer.
Try bending the cable and see if the color comes back.
You can purchase new 15 pin ends at Radio Shack and solder a new
end on the cable.
2. A bad solder connection at the socket where the CRT circuit board
plugs onto the back of the picture tube. Just resolder the connections.
3. A bad video driver transistor or driver IC that supplies the color
signals to the picture tube. Measure the voltages on the three color
signal connections on the picture tube socket. The missing one will
usually be very different.
Hope this helps...
John