Harlequin has been used as a general term for solid tricolors since as long as I can remember, but in all of my years breeding tricolor holland lops, I have never seen a true harlequin holland lop that would be showable if the standard allowed for that color in the breed. Pictured above is the illustration in the SOP for the color Harlequin. Distinction between the colors is the overall theme for Harlequin. That being said, the goal in tricolors is to get broad distribution of the black throughout the orange blanket, which is really the opposite of bars or bands, and this is the reason for solid tricolors in Holland Lops to not have the actual harlequin appearance as written in the standard for Harlequin rabbits. This is true for any breed with Tricolor as a showable color, since none have a solid counterpart. This means that solid tricolor has NEVER had an accepted term or standard and Harlequin has been used colloquially, but never officially. It is important to note that there is not a broken version in the Harlequin breed either, so Tricolor and Harlequin have never been actually associated with each other in any breed. Brindle is the first time a term has been approved by ARBA for a working standard that would allow for a showable solid version of tricolor.
All that being said, it has of course been discussed in the past about adding Harlequin as a showable color in breeds like Holland Lops where tricolors are showable. Harlequin pattern is defined as, "Six markings and their alterations-ears, face, chest, forelegs, rear legs, and body." pg 12 SOP. According to ARBA, any breed that applies for the COD for harlequin would have to adhere to the actual color standard already in effect for the Harlequin Breed. I want to take a minute to talk about some of the reasons that would be problematic. The actual COD process would be complicated and would take (by my estimation) a minimum of 10 years before even being able to apply for it. In order to have true harlequin colored Holland Lops, it would mean outcrossing to the actual harlequin breed. As someone who has done outcrossing to other breeds for the sake of color in Holland Lops, I know firsthand that it is a difficult process and requires a lot of balance in which you want to line breed quality Holland Lops as much as possible to ensure showable type, but doing so leads to a loss of the color you are going for, so you have to find a very delicate balance between breeding for color and not accidentally setting type of the incorrect breed in your Holland Lop line. While I do not have experience with the Harlequin breed specifically, I have spoken to multiple Harlequin breeders and it sound like the pattern breeding is very similar to that of the Dutch breed, which I do have experience with. When we were breeding Dutch, in a fairly average litter size of 8, we would typically get 0-3 showable (not great marked, just not DQ) Dutch rabbits born. With the smaller litter size in dwarf breeds it would be possible to go several litters without getting anything showable based on pattern, which would not only be a huge obstacle in getting the COD and going through the process, but also for breeders later when the color is showable. Breeding for color or pattern is very tricky and it requires a LOT of cage space and a willingness to grow out everything you produce, which is very time consuming and very expensive. Whenever you would get to that point where you have consistent type and color, then you apply for the COD and hope that color sticks through the next several years of generations. You would also have to go through the entire COD process, which is of course also very time consuming and expensive, but that is also true for brindle, so I will skip that for now. After all of that work to get Harlequin approved as a showable color, the patterning required for show would not be compatible with any current show line tricolor holland lops, which would be really annoying for breeders like myself who have been working so long on trying to have good color distribution on show line tricolors. Tricolor itself is still a bit taboo (though way less than it was a decade ago) in the show setting, and losing the 4 points for color could easily be the difference between earning a leg and ending up in second place for a tricolor rabbit, so not breeding for the good color distribution and working towards bands in tricolor is not something a tri show breeder would want to do for that reason, but it would be necessary if the lines were to be used in conjunction with a Harlequin color line. They just wouldn't benefit each other overall, so it would be a lot of work to get Harlequin approved, with the actual goal of solid tricolors being showable not really reaching fulfillment. While harlequin could theoretically become showable in Holland Lops, it's not really worth the time, money or effort that would go into it, especially now that we have the potential of the Brindle standard and the progress it is already making to become showable in Mini Rex.
So overall brindle just makes more sense for actually being solid tricolor, but I also like the idea that it could (maybe one day in the very distant future) open up to chinchilla based japanese (generally called magpie) to get a similar COD eventually.