Thanks for your replies!
It cannot be the S-Video cable, since going directly to the Sony Video monitor from the S-VHS player with the same cable I get the colour information. The chroma channel gets somehow lost, when I go into the AJA io.
I tried it using Final Cut and its settings as well as with the Control Panel – no colour on the S-Video.
Somehow strange that in the control panel I can chose in the Input Menue “S-Video” but only have the choice between Component Output options…