is Black. However the Cathode Ray Tube [CRT] type monitors have three electron guns, one aimed at the Red phospurs, one at the Green, and one at the Blue. All of the colors are created from combining those colors [colours. speaking proper English LOL].

You may have lost the Red gun in the monitor or the red signal from your video card, or a bad cable between.

Since you say that the sytem is old, it wouldn't be too unusual to have the red gun path in the monitor to go bad. Check the cable, try a different video card. Trying to get into a CRT monitor to repair it is a difficult and dangerous job. rather high voltages inside that case.