I recently had almost the EXACT same experience as you. The project was to archive a bunch of 3/4″ tapes using the client’s VP-5040 u-matic player. Playback on the studio monitor looked fine, but the signal to the Kona 3 card (using the deck’s composite output) was black and white.
At first I thought it was the unstable signal, so I bought a cheap TBC (Datavideo’s TBC-1000) which definitely helped stabilize the signal, but it didn’t restore the color. I’d never experienced a monitor being able to display color that the capture card couldn’t so naturally I assumed there was something wrong with the Kona. Many, many tests later I concluded that the card was fine and the only possible culprit was the deck.
I rented a VO-9800 and voila– color!
The captures are as steady and clear as the source material allows.
I recommend trying another deck.