I thought the TCS-34725 RGB color sensor (https://amzn.to/2M0ZqzB) might be the solution, but it’s not. I printed a little slide carrier in black, so I could cut out the ‘scopes light when out of range. That worked great, until the next day and I had sunlight.
The sensor is good at picking up reflected light off the top of the slide. Which is fine for other applications, but no good for us. (Unless we run the ‘scope in a dark room.) I’m not able to get clear in/out of range readings with the reflected light getting in the way.
I still may use the sensor, as a very-rough “print preview” of the scan. It does change values as I move across a sample. However, if ambient light changes, you get shifts in the color. So it’s not very repeatable.