Reading the three numbers
Picture a block of pixels 4 wide and 2 tall. The first number is that width, so it's almost always 4. The second is how many color samples the top row gets, and the third is how many the bottom row gets. Brightness never changes, because every pixel keeps its own. Only the color is shared.
- 4:4:4 gives every pixel its own color, so nothing is saved
- 4:2:2 shares one color between each pair of pixels in a row, which cuts the data by a third
- 4:2:0 shares one color across each 2 × 2 block, which cuts the data in half
The quick check is to add the three numbers and divide by 12. The sum is 8 for 4:2:2 and 6 for 4:2:0. Divide those by 12 and you get 0.67 and 0.5, the share of the full 4:4:4 data that each one keeps.
What the savings buy you
Dropping color detail is the cheapest way to make a mode fit a cable. Take 4K at 60 Hz with 10-bit color. Full color needs 15.7 Gbps, 4:2:2 needs 10.5 Gbps and 4:2:0 needs 7.8 Gbps. HDMI 2.0 carries 14.4 Gbps of data.
So full color at 10 bits doesn't fit an HDMI 2.0 link, while 4:2:2 and 4:2:0 do. That trade is how a mode like this gets through.
DSC is the other way to fit a mode. It compresses the whole picture instead of cutting color detail, but both ends of the link have to support it. The Display Bandwidth Calculator shows each chroma option for your resolution, refresh rate and cable.
Where you can see the difference
Subsampling shows where color changes sharply from one pixel to the next. Saturated colors that meet at an edge lose detail and bleed into each other. On a desktop that means colored text, thin colored lines and small icons. Dark text on a white page mostly escapes, since brightness keeps its full detail.
Movies are the easy case. DVD and Blu-ray discs, most H.26x video and the common JPEG and WebP files already store color at 4:2:0. Sending a 4:2:0 signal to a TV for those costs nothing, because the color was that coarse to begin with. For a living room TV that's a fair price for a higher refresh rate or HDR. For a PC monitor, keep 4:4:4 if the link allows it.
If text looks soft at a high refresh rate, check the color format in your graphics driver. A mode that only fits the link at 4:2:2 or 4:2:0 can end up there without any warning.