I am looking to input real-time audio spectrum analyzer signals...
I would like to use a specific bass frequency value, for example 200Hz from the X-axis of the spectrum analyzer, then an amplitude threshold value of the Y-axis, for example -6dB to be used as the ON/OFF switch for the LED, and finally multiple loudness values of the 200Hz frequency on the Z-axis of the spectrum analyzer which would contribute to the LED's intensity or brightness.
I know what a spectrum analyzer is, but that's confusing... It's OK, as long as you know what you want to make.
There is a nifty 7-channel spectrum analyzer chip called the MSGEQ7. If you can get-by with 7 frequency bands or less, and if the fixed-frequencies are OK, that's the easiest way to go. (I've never used that chip and some people have trouble with it, but it was me I'd try it. If you do try it, buy your chips from a reputable supplier. There are apparently lots of fakes or rejects floating around.)
If you need more frequency bands there are FFT or FHT software libraries.
If you go with FFT or FHT, you'll need to bias the input (because the Arduino can't accept or read the negative half of the AC audio waveform). That's just 2 equal value resistors and a capacitor. If you use the MSGEQ7 you don't have to worry about that... It's designed for regular AC audio.
Both of those will work with approximately line-level or headphone-level audio signals.
Depending on what kind of lights/LEDs you're using you'll probably need some kind of drivers or relays, etc.
I'm not tech-savvy but I believe this is a cool concept worth pursuing if it is possible.
It's certainly possible! If you search YouTube you can find all kinds of spectrum analyzer (and other lighting effects) built around an Arduino.
As with any project, take it one step at a time and work on the input (frequency detection) and output (lighting control) separately before you put everything together.
And when you write your software add just one or two lines at a time, testing as you go (after you get enough code to get started).