It's a complicated subjective topic, I don't think you necessary need to separate two categories as often you could combine foot fetish, stockings and office wear.
In any case if you are chasing popular trends then probably the longest lasting and most successful is Kemonomimi, i.e. characters with animal characteristics such as cat, fox, etc. Just see for example the vast majority of the most popular running virtual youtubers today are Kemonomimi, such as Gawr Gura, Inugami Korone, Nyanners, Kiryu Coco, Ironmouse, etc. None of these very popular characters are just regular humans.
If you try and cater for only a few people then you will greatly limit your sales. It would be better to just have a range of images and let the person decide what ones to color
Also I would love to see what artwork you intend to use as if its easily found Azur Lane artwork like that then nobody will buy it as its very doubtful you managed to get both Tsubasachyan and Azur Lane permission to use the artwork.
(Yes I know you're using them as an example but considering all three messages you've posted on this site are about being a publisher for coloring books, you must have some in-house examples to use without using someone elses images)
This is a very tricky question. Not only is it very subjective, just knowing which of the two is more popular doesn't necessarily translate into a better fit for a colouring book. Looking at the examples, the ones featuring feet seem harder to colour due to large areas that only differ in shading. I would probably fail horrendously colouring those in, whereas I think I'd get better results for the others. (Though I'm not the target audience, so my opinion doesn't matter)
Honestly I would try and figure out what your target audience prefers (to colour!). Perhaps hold a survey? Or focus on a colouring book that features more generic images, but mix a few of these in. Then try and figure out which were the favourites. Again, using a survey or perhaps with a contest.
But to not leave you empty handed, I've quickly scraped some numbers from the Danbooru2020 dataset (sfw link, don't worry).
Total 2019 (%sfw)
Total 2020 (%sfw)
Plenty of caveats, but as you can see @CommanderZx2 was right on the money that kemonomimi is considerably more "popular". Not only that, it seems that the tag 'feet' tends to occur more in NSFW images compared to the other two tags. For the record, 77% is also the global percentage of sfw images on the site.