That's really interesting, and is definitely why they broke! For future reference, here's a list of all html color names: https://htmlcolorcodes.com/color-names/
I just had a script go through the DB though and convert any groups using color names to their hex values. Thank you for reporting (and debugging) this! Let me know if any other groups are still having the issue.
As for why this behavior changed, NodeBB added stricter color code validation on August 5, likely as a security measure, which defaulted non-hex values to #000000 (all black, no color). Previously, the these color names were passed directly into the CSS of the page, which can cause problems.
More background info on how to work with these color hex codes, if anyone reading is interested:
A hex color has three parts: #FFFFFF (RGB, red green blue in that order)
Each component is actually the number 0 through 255 in hexadecimal. (00 is 0, and FF is 255)
So MediumSeaGreen for instance, is: #3CB371
3C in hex is 60 in decimal, aka 60/255 or 23% red (aka dull red)
B3 in hex is 179 in decimal, aka 179/255 or 70% green (aka bright green)
71 in hex is 113 in decimal, aka 113/255 or 44% blue (aka medium blue)
Note: Hexadecimal is a strange concept if you haven't seen it before, but all you really need to know is you can use into sites like WolframAlpha to convert them
Hexadecimal is base-16 (sixteen possible values for each digit), whereas our normal decimal counting system is base-10
In short, base 10 goes from 0 through 9 for each digit, and base 16 goes from 0 through F instead
A is 10, B is 11, C is 12, D is 13, E is 14, F is 15. The alphabet is used to make the extra digits.
In base 10, you count: ... 7, 8, 9, 10, 11, 12, ... (It wraps around after 9, and becomes 10 with a new tens place)
In base 16, you count: ... 7, 8, 9, A, B, C, D, E, F, 10, 11, 12, 13, 14, ... (It wraps around after F, and becomes 10 with a new sixteens place)
Anyway this became a longer post, but I wanted to explain it because I remember also stumped by these color codes for probably way too long when I first started seeing them on the Internet. And I hope this explanation doesn't just make them seem more confusing... The list of full color names mapping to these codes is actually really old, like from the 80s on early computer systems, and it was re-used in HTML in the 90s.
For even more info, this Vsauce video goes into much deeper detail about how screen pixels work using RGB subpixels to create other colors. The hex percents (eg. 23% red, 70% green, and 44% blue for MediumSeaGreen) directly control the brightness levels of these subpixels, creating the optical illusion of one solid color:
