Reflective sensors' primary task is to distinguish between black and white surface. However, ML-R sensors, like MRMS reflectance sensors 9x, CAN, analog, I2C (mrm-ref-can), ML-R Reflectance Sensor A (mrm-ref-a), and other are constructed in a way to be able to detect a highly reflective surface, like metal foil. Black reflects light poorest, white much better, but a metal foil even better. The key to success is to have a right transistor, and at a right distance, so that light reflected from a white surface doesn't saturate the transistor. For example, if the readings upper limit is 1023, and we have the following results, that will indicate saturated white:

• Black: 100
• White: 900
• Foil: 910

In this case, it is not possible to reliably distinguish between white and foil. However, the next example is much better:

• Black: 100
• White: 600
• Foil: 900

The window is much bigger and here silver can be easily detected. How to achieve the second readings?

• Move the sensor higher.
• Transistors are not the same, neither illumination is. Find the one with the lowest readings. Most probably one of the 2 edge transistors as they have only 1 LED illuminating them, instead of 2, like all the rest.
• Use a separate sensor that is put higher. This options has advantage of reading analog values constantly. If You have a sensor like MRMS reflectance sensors 9x, CAN, analog, I2C (mrm-ref-can), You can use it either as digital or analog. It is possible to read digital and analog values at the same time, though the sensor's response time will increase.

Task: read silver reflective foil.

No code here. Just put the sensor on the right distance and locate the transistor with lowest value.