They use dithering to get to the other colors. https://www.tenforums.com/drivers-hardware/116514-acer-nitro-5-an515-51-laptop-display-bit-depth-6-bit.html
I have bought an ASUS N705 laptop that was suggested to me on this website. It looks good, altought I am before a SYPDER5 pro calibration process so the colors are not "fixed" yet. However if I go to the Monitor Settings and Special Display Settings, it gives me an information that the Color Depth of my screen is only 6bit instead of 8bit. Is that normal?