I get a sharp black/white picture when I hook it up to an Extron IN1508 Presentation Scaler, or the Retrotink 2X – whether directly to their inputs, or through a switch.
Knowing that composite over S-Video is, stupidly and unfortunately, a thing, and that the IN1508 will only interpret luma from composite over S-Video, resulting in a black-and-white image, it sounds like chroma isn’t connected.
So, if the IN1508 isn’t showing color, and your VP-720DS (Does this say whether it’s receiving CVBS or YC?) and el cheapo converter are showing color, it sounds to me like composite is being sent, not YC.
Please post photos of your mod work so they can be inspected and compared to the instructions you linked. In the meantime, I’d doublecheck my work, particularly around where the instructions say to remove that resistor and replace it with a switch.