You have the voltage to frequency converters - what is the range of frequencies?

When you speak of calibration - what do you mean? The time the sensor takes to measure the color?

I see no problem with polling - you either need to measure your sensor frequency quickly or have it measured as a background task.

One would think that as each object passes the sensor, you'll see a change in the measured value from black to whatever to black. This might allow you not to have a trigger.

Omron have vision system that do what you want methinks - they're not cheap (>USD$10,000)