I got to wondering( it's a bad habit) how commercial labs handled this process. I'm reasonably sure they didn't do this for each frame of a roll of film.
They did if you wanted the best quality, and paid for it.
Generally speaking there were two classes of commercial labs - amateur labs (catering to amateur photographers) and pro labs (catering to pro photographers). Pro labs of any decent size would use video analyzers, as mentioned by Matt. 1970s or so these would be Kodak VCNAs, and later PVACs, each costing about as much as a small house. A skilled operator could dial in to within 5CC of color which was generally good enough for "proof" prints, and perhaps a lower cost pro product. A grade higher pro product would be rebalanced by hand, if needed, etc. Higher yet would be critically evaluated, and you would pay for it. At the highest level the printer might be making "masks" to make subtle improvements.
Amateur labs used the sort of machines as per odell1618photography. The earliest ones used a 3-color sensor under the negative that would automatically control the exposure (including color). (It doesn't work like the light sources you are reading about.) These sensors would integrate the total light of each color, then shut each off as it was satisfied. The idea was that the average scene would roughly average out to a lightish gray tone (much like an 18% gray card). (Obviously certain scenes don't follow this rule.) First generation 1-hour lab printer machines worked like this; the operator had override buttons to bias the exposure and color as they saw fit. The biggest breakthrough in this realm was the Agfa MSC mini-lab printer which introduced the first scanner, allowing the machine to do much better color adjustment, as well as find the negative edges, allowing automatic film advance.
For you, you should listen to Drew. As a note, I've worked with dozens of professional color correctors - not printers who also color correct, but people who spent the better part of the day calling out corrections to test prints - and none of these people ever used the viewing filters. Except when a member of management was second-guessing their correction, then the filters become a management education tool. Best way to learn color is probably to print yourself a color ring-around - a set of color variations in a couple of strength levels - then let this be your guide. The viewing light can be pretty critical; absolutely do not use CFL bulbs, nor any version of eco-friendly fluorescent. For a reality check, view by window light.
As a note, all of these systems relied on having a system adjustment known as "printer slope." This was set up using what are called "printer setup negatives," commonly known on the internet as "Shirleys." This deals with color shifts, largely attributed to paper reciprocity failure, as print exposures change due to over and underexposed negatives. It won't be that big a deal to you, as you'll be hand balancing everything based on tests. But it does hurt to try to keep a fixed exposure time as you make your corrections.
