Hi. I’m trying to switch the mod settings from using pipe colors to using liquid colors, but I can’t do it because the selection menu is inactive.
I also noticed something else: if you build a regular pipe and an underground pipe, then use the upgrade planner to set the color “water” for each of them, only the straight pipe changes. The underground one doesn’t — you have to manually select the “blue” color for it.

Could you tell me how to adjust the mod settings so that the pipes use the liquid colors?

You can only change those settings from the main menu (they require a game restart to take effect, so the game won't let you change them while you're playing a save). But if you go out to the main menu, then go into mod settings, you should be able to toggle any of the options, and enable the fluid color recipes :)
For the planner tool, there are some per-player settings that you can change - but I'm not sure how much those will help since I'm not exactly sure what's going wrong there. I'll have to do some testing to try and reproduce it.

Somewhat related to op's issue.

I get really confused with so many colors. So I wanted to only allow the fluid pipes
(enable only water, oils, etc and hiding red, green, blue, etc as well as the pride ones {I was hoping for flags there})

But I couldn't do it. Ended up trying to change the source code and the only way I found was to just delete the pipes I didn't want :/

What am I doing wrong?

Maybe having the fluid's icon over the corresponding pipe would solve it

Thanks

In the Startup mod settings tab, you are able to independently show/hide the set of rainbow colors, fluid colors, and pride colors. By default only the rainbow colors are shown, but you can turn them off and turn on the fluid colors if you just want those.

And funny that you mention it - while I was working on the most recent update I was considering adding a little icon for the fluid color pipes showing the fluid color they're based on. I'll poke around and see if I can get it looking good for the next update.

Also, @cheev, I was not able to ever reproduce the issue you described with the planner tool only working for straight pipes and not undergrounds. If you could please share a modlist or some steps to reproduce the issue I would be happy to take another look and get that resolved for you.

I messed around a lot with the configuration, but was not able to activate only the fluid color.

Maybe it was because at first I tried to use the pride flags?

But, when trying to modify the mod myself, there are two or three places where tho mod decides what to show/hide.And I believe that there's a logic error that won't fully allow for my particular combination of settings

Show fluid colored,
Hide rainbow colored,
Hide pride colored.

Right now I'm in a state where the recipe for the pipes is disabled because the tech is not getting triggered. So I copy paste the force enable recipe commands for each recipe 😅