I think it's just changing tastes and marketing.
I have a collection of vintage Highland Dress catalogues and statements like this are seen in some of the earlier ones:
"The dyes used by the Highlanders in colouring their tartans were produced from lichen roots and Highland plants, the colours so produced being mellower and more lasting than the somewhat harsh tints of 19th-century aniline dyes...it has now become possible to reproduce with a certain new process the old light and delicate shades yielded by the vegetable dyes which often disclose unsuspected beauties in the design of tartans which have appeared commonplace when manufactured with ordinary aniline dyes...
Indeed this type of tartan has deservedly become exceedingly popular…." (1936)
So in the 1930s the so-called "ancient" colourway was becoming "exceedingly popular" though I don't think it had all that much to do with actually recreating the original appearance of 18th century tartan.
Then in the 1940s Dalgleish came out with their "reproduction" colourway which supposedly recreated the look of tartan buried for 200 years in a peat bog or something. Once again I think it was about marketing more tartan rather than historical accuracy. Lochcarron named their version "weathered" colours.
House Of Edgar has their "muted" colourway which is quite lovely and transforms yet again the appearance of familiar tartans.
Perhaps the most recent iteration of this process has been the sudden (and to me inexplicable) popularity of grey-scale tartans. Seemed for a while all the kilt hire shops had "grey this" and "grey that". Tartan has always been about celebrating colour- why suck all the colour out of it?
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