Heck yes on the middle layer! Per key RGB was a bit tricky and I think I understand it, but the entirety of my code is in my repo. Definitely reference that. With the assumption that you have QMK set up:

1. Enable RGB matrix lighting and optional tap dance for some shortcut fun in rules.mk
2. #define SPLIT_LAYER_STATE_ENABLE, #define EE_HANDS, and #define RGB_MATRIX_KEYPRESSES in config.h
3. State your safety net for max/min led range so it doesn't loop infinitely and use rgb_matrix_indicators_advanced_user to set up some if statements.

An example - on my layout, I have E, S, D, F as my arrow keys on my second layer.

if (i == 8 || (i >= 14 && i <= 16)) {

RGB_MATRIX_INDICATOR_SET_COLOR(i, 250, 55, 55);

}

With this if statement, i represents the key codes I'm targeting (see image below). || separates each range as "or". So if my range is (8) or (≥ 14 and ≤ 16), i then is defined with that RGB color code. Basically pink. Or you can say (i, RGB_PINK) or any other defined RGB string you can find in the QMK documentation.

Everything after the RGB LED Setup divider in my code is just there for per key rgb on any layer above 0, plus my simple tap dance command to double tap the shift key and turn it red when caps lock is enabled. DM me if you have any questions!

Edit, also! Here's the LED map for each key for when you define your ranges. Hope this helps.

https://preview.redd.it/yvldam3nthja1.png?width=640&format=png&auto=webp&v=enabled&s=4fbb9a7ebfe0110cfd55a34678d4fec909a2305c