I finally found those very interesting documents :
- SaikoLED — How to convert from HSI to RGB+white.
- http://www.mirlab.org/conference_papers/International_Conference/ICASSP%202014/papers/p1214-lee.pdf
-
http://web.archive.org/web/20101008153429/http://www.nouvoyance.com:80/files/pdf/Adding-a-White.pdf
They show several algorithms that can be used to convert a RGB/HSV represented color to a RGBW color, improving some brightness properties of saturated colors, reducing power consumption and making some colors more natural, especially pastel colors.
I think that was what I was looking for! Thank you for your answers!