I definitely don't want to hide projects that aren't possible on a given planet - the player should be able to view those projects and the requirements, and why they can't be done. Similarly, hiding planets where no projects can be started would prevent the player from viewing the possible projects and their requirements. I know most players won't read a lengthy forum post explaining the terraforming system, so I think showing players the projects and their requirements (even those that can't be done) in-game is the best way to teach them how the system works.
Sure, people have to get from somewhere, how the mod works. But in my opinion always showing all options does the opposite, because it is distracting or confusing.
Of course there is a difference between things that can't ever be done and those which can't be done because of missing prerequisites. The former should just not be shown, because that is how the game does it everywhere else (if you can't trade with a market because you're hostile, the entry isn't there; if there aren't any messages for certain intel tabs, those tabs aren't shown; aso.). E. g. if a climate is already "mild" there is no point in still showing the entry. If a planet isn't irradiated, toxic, without right density, with extreme weather there's no point in showing the respective entries. If all atmospheric conditions are optimal, hide the "atmospheric conditions improvement options" menu entry (or show it greyed, with a tooltip of paraphrasing "all done"). The same for resource improvement. If a resource is already maxed, no point in showing the option. If all resources are maxed, hide the "resource improvement options" (or again show it greyed out). The same with planet type change options. You can't change a hot planet into a frozen world.
This could also be a kind of way to find "done" planets. If a planet is habitable and there aren't any atmospheric conditions and resources to improve, only show it in the "all planet" list.
Regarding changing the color of the entries, it might be confusing to players why an entry is a certain color until they've used the menu a lot and learned the meaning of the colors, but by that point they'll already know the requirements for the different projects.
That's for the actions that can't be done because of missing prerequisites. It doesn't necessarily have to be red, green, yellow aso. But Colorcoding is also done in the game in a lot of places. If you don't have enough story points or credits or resources to perform an action the entry/button is there, but greyed out. That's also a form of color coding and pretty universal. Usually when you try to select/activate such an entry anyway, that's when you get that "not possible sound", a message or tooltip why it can't be done.
I'm not sure printing which buildings are missing before the player enters into a menu is a good idea. It will clutter up the text panel and might be confusing if there are projects with different building requirements.
That was an alternative to color coding.
My point is usability. Currently there is A LOT of clicking and going through menus only to find out that most of the actions CAN'T be performed. This becomes especially tedious if you have 10 or more colonies, where remembering what you already did on what planet or want to do is pretty much impossible (unless you're some kind of genius in memorizing stuff). If I come back to a game after a few days, it becomes a game of trial and error going through almost every menu for almost every planet to "learn" where I was when I left off.
Don't get me wrong. I'm not criticizing the functions of the mod. In my opinion Startsector doesn't feel complete without it and I'll use it in any case!