You’re right; you need to put some science into it.
Start with a calibrated test signal. Here is a direct link to my test pattern mp4 file which is free of YouTube’s downsampling:
Next you will need an eyedropper program to check the colors.
http://instant-eyedropper.com/
You can either view the test pattern in a browser or, preferably, download it and view it with a player such as SMPlayer which I use, or VLC or yes, Windows Media Player. CAVEAT: Chrome and Microsoft Edge are the only browsers I’m familiar with that are color-accurate. Firefox is a big disappointment in this regard.
I have found that the primary and secondary color patches will be +/- 3. The actual values are 16 and 180. 16 should be familiar to you as it is “black” in BT.709. So yellow would be R-G-B 180-180-16.
3 / 255 is a little over 1% if you want a percentage.