You need to switch the S-Video adapter cables around. You are using the S-Video adapter cable designed for the input signal on the output of the Power Macintosh AV card. If you examine the adapter cables, one is marked IN and the other is marked OUT. You can only get a color signal when you use the OUT adapter on the output of the AV card.
This symptom appears only on Power Macintosh AV computers and not Quadra AV computers since the Quadra AV card has a composite output source instead of the S-Video adapter cables.
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