Assign custom attributes to each individual vertex

Question

Problem:

I'm trying to create a row of vertices to render as Christmas lights. Each vertex has its own x, y, z values, but I would like to add a fourth attribute to use as brightness.

Here's a simplified visualization of what I'm trying to achieve:

Each vertex has its own custom attribute so I can assign it as a brightness from 0 (dim) to 3 (brightest).

Is there any way to add a fourth attribute that doesn't involve vertex groups? Exporting vertex groups via JSON gives me four arrays of a binary nature:

"Group 0": [1, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0]
"Group 1": [0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0]
"Group 2": [0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 1]
"Group 3": [0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0]

What I need is just one array because the values won't necessarily be in discrete integers:

"Groups" : [0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3]

Question:

• How do I assign a custom attribute to each vertex via the GUI?

I've seen answers that need use of the BMesh module, but I don't know scripting, and I need to edit each vertex individually (hopefully via the GUI).

I saw the Custom Properties dropdown, which looked very promising, but I cannot assign individual values to each vertex, it only assigns a single property to the entire object.