Jois:
What you probably saw was a “color key.” Color keys were (and probably still are, to some extent) a very popular and accurate pre-press color proofing system. Here’s how it worked.
Original artwork, let’s say a photograph, is placed in the giant camera I mentioned earlier – it’s called a process camera if it is capable of shooting color separations. The photo is shot 4 times (3 of those times using color filters) to “capture” the color components of the original photo. The four negatives are the color seps: one will be used to print yellow, one cyan, one magenta, one black. NOTE: All these negatives are themselves black (opaque), with transparent dots.
(Extra credit and will not be on the final: Those first three colors are known as the “subtractive primary colors”; their dots overlap in varying amounts on a white page to – theoretically – create any color in the spectrum, including black. That is probably (???) why black was optional when you saw your aunt. The reason black was usually (these days, always) used is because, here in the real world, the Y+C+M used alone combine to create a crappy, muddy sort-of-black.)
Now, if everything has gone perfectly (ha!), you could take those 4 seps, burn 4 metal printing plates, mount them on the press, and run the job. But chances are your client would come over to ok the job and see a big ugly hair that accidentally got stuck on the original photo and is being printed at a zillion copies a minute… or any of a thousand other reasons the job is not right, and needs to be fixed. A very, very expensive proposition.
So, what is needed is a way to “see exactly what the job will look like” before you burn plates and put it on press. That is called a pre-press color proofing system, and the most common was/is the aforementioned color keys.
The negatives are inspected for obvious flaws (dust, scratches, the hair, etc.) which are opaqued out. Then, thin transparent sheets of mylar – each coated with a thin layer of light-sensitive Y, C, or M emulsion – are exposed in contact with the negs and developed. The result is a set of 4 “positive” color key sheets which, when stacked on top of each other and positioned perfectly (“registered”), create a full-color image that will match what the printing press will create.
This is probably what your aunt worked off of (and what the client would need to okay). It’s easy to catch tiny errors in the negatives at the proofing stage. And, if she could fix the errors by hand she would save the time and expense of shooting the job all over again.