For upgrading to Mojave, I had to change my old Nvidia cheap card to a new AMD Radeon Metal 2 support card. I have an IPS monitor and did not change it, but from when I did change the card I feel picture quality is much better and colors are very clear and natural! I an an Elec. Engineer and I know colors are sent using numbers to the monitor (Display Port is a digital interface). So the picture quality AFAIK must not relate to Graphic Card, but just to monitor which has not changed. Am I right? Or there is something like color scheme or so that changes my imagination about this topic?
DisplayPort is a digital interface, yes. The actual display signal is sent essentially as 0's and 1's in a packetized protocol that ensures that either (1) what is sent by the computer is what is received by the monitor, or (2) the monitors knows that the something is wrong with the signal and will usually just show blank (black) screen.
In practice there are some other factors playing into it. DisplayPort comes in various versions supporting various sets of features. In addition DisplayPort supports several transfer speeds, so a "low quality" cable might only support slower speeds (i.e. lower resolution, color depth and/or frames per second) compared to a "high quality" cable. Beyond this some transfer modes employ a lossy compression, which is commonly referred to as "visually lossless" - i.e. in the digital sense you're not getting the exact same data before and after compression/decompression, but to the viewer it looks (almost) the same.
That being said, it is not so that the graphics card has no influence on picture quality at all. Nothing could be further from the truth. The programs on the computer are not spitting out DisplayPort packets that the graphics is simply responsible for transferring from the PC to the monitor. The graphics card performs a lot of duties in between the input from the operating system and running applications, and the output to the monitor.
Essentially an old, low-quality graphics card might for example only offer low resolution, low color depth, few color spaces and color formats, etc. when compared to a newer and/or higher-quality graphics card. When you look at 3D (for games or very graphical applications) there's tonnes of other features that the graphics card might or might not support in various degrees of quality and speeds. These can affect the image quality hugely.
Besides the above mentioned, you might also experience better quality simply because you had a non-optimal setup/configuration of your old graphics card. Or it might be because the new operating system install contains better drivers.