So I want to use S3PE or a similar program to access the data file where all of my unique drinks made in a save (or all my saves) are stored so I can mod edit them to look less boring.

The problem is I don't know where the data would be stored. Can anyone help me?

You mean the drinks that come with Late Night bars, right?
Do you want to change their names or how they look like?

I mean the new drinks that my mixologist sims create. I want to change how they look (color, glass, pick, whether or not it's on fire).

Oh. You can learn something every day, I had no idea that save files could be opened with S3PE,
even if it's mostly unknown file formats though. I'm not sure if trying to edit saved data is the best
approach (well, it is to accomplish what you want, but doesn't seem easy to do), maybe you should
concentrate on customizing/tuning how to create new drinks, supposing there are such options.
If I find something I'll let you know.

What I mean exactly is this: Sim creates a new drink, and it looks like a regular drink. So you use some sort of program to find the XML data file that stores the information concerning said drink, then edit this information so the drink's look changes (e.g. it turns green, gets a plumbob pick, is served in a round "romantic drink" glass and is always set on fire).
If it's possible, I'd also like to change the quality so that a "foul" mix becomes better quality.
In another thread, it was mentioned that you could use S3PE to edit a bar mod and change the look of the popular mixes that come with LN, I just want to do the same thing with my custom drinks.

No, custom drink data aren't stored in such a fashion (at least in my knowledge).
This should have nothing to do with the save file. In fact I doubt you can do such a mod.