So you're trying to make a reflectivity meter.
Not that hard.
- get an LED with the colour of interest (materials may have wildly different reflection for different wavelengths of light).
- get a light sensor (as you insist on analog that would have to be an LDR or a photo transistor).
- build it inside a light proof housing, at exact opposite angles to your sample, and start measuring.
Note that you'll have to do calibration with known surfaces to get any useful values out of it.