JimB pretty much covered it.
Standard LED's have a parameter called Vf that has a range. You can buy a bunch of LED's and select them by Vf. Don't worry, it's easy.
The amount of LED's in series cannot exceed the sum of all of the Vf's. Whatever is left has to have a resistor. e.g. (4*2.1)=8.4. R= (12-8.4)/If.
If is the operating current and is typically 10-20 mA. R<=8.4/20e-3 in the case above. Resistors have a power rating and you can use P>(20e-3)*(20e-3)*R
Resistors only come in certain values and power ratings, e.g. 1/8, 1/4/, 1/2 and 1 W
Colors complicate things, because each color has a different Vf. Sometimes you can buy LED's within a certain illumination range.
So, with multiple colors, you can use a few LED's and a single resistor. If you string multiple colors together, they may not have the same brightness.
One resistor per LED generally solves that problem.
This datasheet
https://users.ece.utexas.edu/~valvano/Datasheets/LEDorangeHLMP-D401.pdf shows how colors have been binned and the variations of Vf for different colors. There is also a viewing angle spec.
Here's a blurb on how to read the datasheet:
https://learn.adafruit.com/all-about-leds/the-led-datasheet
12 V replacement LED's are typically made with a series/parallel arrangement of LED's.
Its not that hard to understand,
One simple way to do LED's is to wire every other one and alternate the power. it tends to make them look moving.
A much harder way is a "color organ" where each audio frequency band is shown in colors and the intensity by the amount of energy in each band.
A "pattern generator" is complex.
"relays" can be used to use a lower current source to turn on a high current device. One typical application is the "horn relay". It uses a low current contact in the column and activates a higher current load.
Your 1959 could have positive or negative ground, LED's don't like to be installed backwards. With more than 3 in series, they will likely survive a reverse application of 12 V. A LED/RESISTOR sin series will likely survive.