What is the gpu data depth on NANO and Can I adjust the HDMI output image parameters?

I used the Nano’s HDMI display and couldn’t display all the gray-scale graphs.
I use the same monitor and the image to see the whole gray-scale on another PC.

I wonder if the nano’s GPU memory data and HDMI data depth is 8 bits ?
And if there any way to adjust output image to make gray-scale display equalization?

The default color depth is rgb 24bpp case and with limited range. Which means only 16~235 color range is supported.

Are you saying that some color bar is not visible over HDMI?