That's because DisplayPort on both AMD and Nvidia uses Full RGB as default
HDMI and DVI is a different story and even if the RGB range is set to full RGB on both colour can still be different depending on colour temp etc both brands choice is for default.
Its been years since I owned an Nvidia GPU so am not in the correct position to compare either on this front.
edit
This debate will never be ended
We all have are own taste in what is the correct colour, brightness/backlight everything
Hell some people even like to keep the shop floor presets on TVs for god sake LOL