In the end, this was the most helpful resource:
Hi, recently purchased a Canopus ADVC 110 to convert VHS to digital, and I'm having a problem that maybe someone here can help solve.
I've hooked it up to a JVC BR-S500U via RCA for the audio and S-video for the picture, and what is happening, over the course of a tape playing, is that the picture will be in color for a minute or two, then switch to black and white for maybe ten minutes, then switch back to color for a while. Picture quality is generally good; it's just that I'm not getting color all the time. Does this sound familiar to anyone? It's definitely not loose cables, and I've never seen a VCR do this before.