Darren -
The light gray speckled tub, at least I think, seems more of a super light beige, or maybe a pale taupe color? This color tub was used in Kenmores (I don't know about Whirlpools!) in the upper ended models, from the mid 1970s until mid 1981. In the lower models of the same period, the blue/gray dark speckled tubs were used.
I had always noticed that there were two distinct tub colors, but never gave more thought to them then that, until one day a few years ago, and I did some part number research. Previously, I thought the same thing you did - that these variations were just random. I was surprised to learn that the each tub color has its own part number, or a number for the standard cap tub in blue gray, and a number for the standard cap tub in light beige (or whatever you want to call that). The same was true for the large capacity versions. In 1978 when the centerpost height was reduced, new part numbers were issued for the same variety of blue-gray tubs, and light beige tubs. I was surprised to learn that these tub color differences were intentional, and clearly planned for use on various models, though Sears seldom if ever mentioned this in marketing materials.
What I found out was that in general, with a couple exceptions, the 70-series machines were the cut-off marker, in that all 60-series and below machines got the blue-gray tub, and everything 70 and above got the light beige. The light beige tub seems to have much more lusterous or luxurious layer of porcelain, so I'm thinking that these were essentially the 1970s version of the fancy white tubs that were so common in the 1960s. From 1976 - 1981, there were very few white tubs, if any, in Kenmores anyway.
In this period, Sears had several 'doorbuster' models, or as I have learned recently, they were called "Promotional" models. These were models added to the line after the original yearly models were drawn up, and were de-contented a bit to be offered at steeply discounted prices.
For example, the 1977 70 series has the beige tub, 4-water levels, and a out of balance relay. The deconted promotional 70 has three levels, no relay, and the blue-gray tub. The same was done in 1979, and these were very good sellers as Sears liked to have their Promotional models on sale often. In 1979, this same arrangement was done in the 80-series line as well, and thus created the 1979 80 series that I bought for my sister used in 1989. It has the highest 80-series model number (meaning it should be the fanciest) but it has the fewest features of that year's 80 series, and again, the blue-gray tub. This machine has been my daily driver since shortly after finding AW.org in 2008. There was a long line of models in 1979, but the most common I've come across is this promotional 80-series.
In 1980, the entry 70 series, which had no dispensers, got the blue-gray tub, and the next step up, which had both softener and bleach dispensers on the otherwise identical machine, got the beige tub, as did everything above it, even the electronic Lady Kenmore.
When the belt-drive received a freshening in later 1981 to reduce water consumption in the large capacity models, the baskets reverted to the more familiar BOL blue, mid-line blue-gray, and TOL snow white (although the BOL blue was used only on the "Regular capacity" machine as they called it, which was the 24-inch machine). This was the year that Sears dropped the 29-inch standard capacity belt-drive. I do not believe this pale beige color was used in KMs after that. During this time, I've not seen any Whirlpools that used anything but the blue-gray tub until the white tubs were back in vogue although others such as Glen may be able to elaborate on the WPs of the same period?
Gordon