I've been working on this for the past couple days, and I've gotten things mostly sorted out. I mixed up a batch of color in shellac using my original recipe. Much to my surprise, it was virtually identical to the colors I've been getting in lacquer. The only difference is the lacquer builds better, so I use half as much dye in the lacquer mixtures. I also mixed up a batch of the same color using fresh dyes that I just bought from LMI. Again, the color is exactly the same as I was getting with the "old" dyes. This was a surprise, and I was assuming the the color shift I've been getting was due to partial evaporation of my old dyes.
So, I've taken a few variables out of the equation. The old dyes I've been using appear to be the same color as the "fresh" ones I just ordered (this is for brown and black only, as I've been using "fresh" red and yellow all along). The colors appear to be the same in shellac and lacquer. I'm spraying the colors on offcuts from the same piece of wood as my original color samples from a year ago, and I have an original color sample to compare to. The only explanation left that I can think of is that LMI's dyes must be inconsistent from batch to batch.
Anyway, I would be very frustrated by this, but I'm starting to like the yellow I've been coming up with. I sprayed a burst on one of my test pieces last night and it looks really nice, so I think I'm just going to use it and go on.