You don’t have to convince me that background colors are helpful and desirable, based on my Aug 2020 thread for AoE2, which is pretty much the same idea as AoE4 icon background colors:
But that, I feel, is a different discussion anyways, and only helps a tiny bit. Large groups of icons all share the same color backdrop. The root of the icon issue in AoE4 for most, from what I can tell based on years of discussion, is the gold 2D icons everywhere. And no amount of background colorization will fix that.
I’ve given many thoughts on this topic over the years in the forums, beta, and surveys. At this point, since we’re still in a land of 2D gold-colored silhouetted icons, I thought a traffic sign graphic might help. Sometimes visuals help communicate better than words.
While you observe, apparently, an inordinate amount of misrepresentations of icons, I’m still waiting for a good UI/UX reason for why the current icons are the greatest way to make them. Sure, some players may prefer them because they’re sleek and modern-looking, and that’s fine. But from an actual usability and gameplay standpoint, they’re not that great, and are a devolution from AoE2’s more immediately recognizable icons… or other UIs that incorporate colors, for that matter. And don’t fit the Medieval setting very well.
The Age franchise rewards quick clicks, and the efficient/effective management of your empire. The less time you spend trying to decipher the UI and fiddling with it the better. How is it a good thing to spend more time looking at the UI than necessary, simply because every icon is colored gold and is a simplistic 2D stylized silhouette of things?
From an old thread, I gave these mockups. Wood, food, gold, and stone icons all colored gold:
And a 2D stylized silhouetted variation thereof, to try and help make up for the fact that they’re impossible to know what they are since they’re all colored gold… just like what AoE4 has to do for all their icons:
(Note: Since they’re all econ-related, they’d all have the same backdrop color, so that wouldn’t help things.)
I don’t know about you, but icon colors help me more quickly identify what they are. Thus, less time fumbling over the UI:
Anyways, I’ve given plenty of reasons why the gold icons are inferior over the years. I think I’m entitled to make a traffic light at this point. And that light wasn’t even misrepresenting anything, tbh. Besides, since the traffic light icons of “Stop,” “Slow Down”, and “Go” would all be the category of “Traffic Management”, they would all be in the same group category, and would all get the same background color in the AoE4 world. Therefore, my mockup still doesn’t misrepresent anything 
I have a variety of points in the following threads/posts. I think my main issues w/ the gold icons can be summed up to a handful of things, scattered across these threads and others, I’m sure. You’re free to view or not view… but I’m not trying to misrepresent anything. If I ever exaggerated(?), it’d mainly be due to being in disbelief that the ubiquitous gold icons were ever even a thing, and because they have been the only option for players for so long.
And I’d like faster times in and out of the UI, with clearer intuitive communication (by their art design and coloring) as to what the icons actually are. And these are probably my “most important” wants for the UI/UX right now 
Please help me understand how monotone 2D icons with flat-colored backgrounds (no matter how many color variations you add to the backgrounds) are better and quicker for AoE’s RTS gameplay than simply having more informative and colorful iconography? And if you concede monotone 2D/colored backdrop icons are slower for the human brain/eye to intuitively process or positively identify than what is proposed (even by microseconds), then why does that artistic “theme” (for the sake of having a theme) have more importance than actual RTS gameplay?
Would it not hurt… and wouldn’t it, actually, be good to make the UI/UX quicker and more intuitive for the vast majority of people to use?
For players who cannot see the full spectrum of colors in our world, I have a feeling the AoE4 icons might actually be quite good. I’d think silhouetted representations could prove quite informative for them, and, so, the icons should definitely be kept as an option (assuming this is the case and they want them). Or feel free to even keep it as the default UI for cohesive ‘artistic theme’ purposes, as well, if desired. However, according to Google and Bing searches, a large majority of the population can see all the colors of the rainbow. Because of that, I have a feeling incorporating more color and 3-D depth to the icons (to, say, be more like AoE2), as a 2nd set of icons to choose, would help most players use the UI more effectively and efficiently… and also give them a more pleasing and “of the medieval time” experience, all around.
Logging a ton of hours to get the right muscle memory down to overcome the UI’s shortcomings shouldn’t be what we need to do. But if that’s the desired requirement, then it’s all good. I’m okay with waiting 2, 5, or 10 more years until I buy it for a $1 or $2 during a Steam sale. I’ll probably comment on the gold icons from time to time, though, unless and until there is an option to have AoE2-like icons, simply because it seems like such an easy deal-breaker to overcome.
With all the AoE events/challenges over the years where a bunch of profile icons have been created as rewards, it seems like cranking out a bunch of new icons to actually benefit gameplay would not be that big of deal for their talented design team. A fair amount of work, sure, but hopefully fun work… and with 8 hour workdays, a lot could be done in a week or month, imo