I am trying to make a colorimeter using TCS3200 sensor and white LED light source.

After weeks of research and understanding on how different white light sources work and what are the algorithms involved in calibrating the sensor in CIE1931 space.

Now I am facing problem in getting a reference for getting a calibration curve.

For example, if I want to map R values, then I also need some reference for which I know R values.

I initially planned to use RAL cards to calibrate my sensor, but the RGB values are not shared on their official website and other websites provide sRGB values and not the actual RGB values.

I even thought of printing varying shade of Red on paper, but I believe there will be a lot of unknown factors in this (printer calibration, RGB space used by the software, etc).

So is there any standard source of known colour against which i can calibrate my sensor?

Edit:
My application is more to sense the change in shade rather than getting the actual color of the subject and that's the reason I was thinking if I can get away with calibrating it against some standard color swatch of known RGB values.