If a team wanted to use an optical sensor to measure the RPM of a 4" spinning wheel, would the rules allow the use of a decaled or painted polycarbonate disc as the code wheel? This is a functional component after all.
This should be legal as a color filter/marker. The polycarb backing would count towards your limit. If you want an official answer you will have to ask in the official Q&A. When you do that remember to show R7 a below.
<R7> Robots are allowed the following additional “non-VEX” components:
a. Any material strictly used as a color filter or a color marker for a VEX Light Sensor