From Wikipedia, which spends most of this article’s discussion of dye lots by referencing knitting/crocheting projects, towards the end comes up with this handy little tidbit for other “walks of life.”
Dye lot in other uses
Use of the same dye lot or run number may also be important in other applications, such as: Wallpaper, fabrics for drapes and other uses, carpets, flooring, tiles, etc.
Add to that cellular shade fabrics!
Have you ever tried to match Whites? All whites are not created equal!
How about Blacks? Some are blue, others are a stark black.
Or Reds that are slightly more or less orange.
This is similar to a Dye Lot Dilemma — one batch of dye will rarely match exactly another batch. Add into that mix, for window shades, daily usage, in windows, and you’ve a scenario where “a good match” will be all one can hope for rather than pining for “an exact match”.
Do fabrics fade? Sure. Do fabrics, over several years, undergo a “new and improved” status? Of course, nothing stays the same.
So here is one thing to keep in mind, especially if you are trying to “match” shades: How long your shades have been exposed to heat, dust, sunlight will affect how well a new shade “blends” with the old. This can be compounded by a dye lot difference (especially darker shade colors), as well as new manufacturing processes.