This is very interesting.
Personally for me, too, because I don't want to hide stuff, but colour-code my custom tooltips, and the default options I don't like.
So while I am no expert on this stuff yet, I may share some insight on what I understand so far:
First off, simply changing the opacity for these "G" and "R" things is going to break stuff, which you already noticed (army strength e.g.).
The wiki has a section about this color-coding commands, which it calls "
Special Characters". And until today, I thought these were hardcoded and limited to these few that are given at the wiki.
However, now you just showed me that indeed these are exposed in the code and not hardcoded, and thus open to modding! Thank you for that

You also showed that fundamental tooltips that I also deemed hardcoded, like that CHARACTER_TOOLTIP_DELAYED, are also moddable.
Now, you want to hide opinions, and only those. Plus, you want to color-code the opinion modifier names to red and green, respectively.
Your current approach to doing that is by removing the opacity of the "G" and "R" localization "colorcodes" (as they are called in the fonts.gfx file). This, however, will result in
any localization in the game that uses such a "§G<text>§!" localization to become transparent. For example, the army numbers, as well as e.g. some custom localization in my mod for which I also used the "§G<text>§!" commands.
However, since you just proved that these things are moddable, I believe a better approach would be to do this:
1) Create a NEW entry for all these fonts, e.g. "T = { 0 255 255 }" as a new color. Because you don't really want a new color, of course, you instead create that one with an opacity of zero. So your version would be "T = { 0 255 255 0.0 }.
2) Locate the "Opinion Modifiers Shown" tooltip. I do not actually know what it is called, nor am I 100% sure that it is not hardcoded. But the latter would seem unlikely, given that it is clearly being influenced by colorcodes, which are open to modding.
3) Search for the things that display opinion modifiers in that mystery file, replacing any "§G" and "§R" with a "§T". Ideally, the file would also have something like "GetOpinionModifierName", which gets the modifier name into the tooltip, and which could get color-coded to be green or red then, depending on the modifier. Note that this is all speculation on this point, as I don't know how the code actually looks.
So, that's how I think it should work.
Everything depends then on locating the actual file that includes the opinion modifier tooltip. I did a quick search, but since I don't even know how the tooltip is called, I can't search for it in the .csv files - and manually combing through them would take beyond ages. Assuming still, that this tooltip isn't for whatever reason hardcoded and not actually directly moddable.
It would seem best to me to ask someone of the devs for help, as they would be the only ones I think that know the tooltips name - or whether it is obscured to modding.
@blackninja9939, you are my first victim, since you are frequently around and quite helpful: If you read this post, can you tell us about the bespoke tooltip(s), the ones that display detailed opinion modifiers and their values? Are they open to modding, and if yes, what are they called?