Long post with some history.
Back in TI7 battlepass, kunkka got a shoulder immortal, prize of the saltworn mariner. The first style was blue and the second style was green and the second style is permanently locked unless you completed a quest during the battlepass.
Some players liked the first style blue shoulders more than the second style green shoulders. At that time, we were relatively new to the idea of exclusive cosmetics being permanently unlockable and I think most people just based their decision on whether they preferred the blue style or the green style shoulders for kunkka. If they preferred the blue style they may have chosen not to unlock the second green style. And that was a fair decision, the blue style looks just as good as the green one.
But then TI8 battlepass came and with it, kunkka got his best looking sword, the leviathan whaleblade of eminent revival. And it was green! For whatever reasons, Valve decided that green (rather than blue) would be the superior colour of choice for Kunkka, for future cosmetics.
There was a blue whaleblade sword too but it wasn't a prestige item and it didn't look as good (it has less particles etc) as the green one.
Unfortunately for those who preferred blue kunkka, the green leviathan whaleblade looks amazing with the green style shoulders but looks horrible with the blue shoulders. And suddenly all the people who chose blue kunkka realised they made a terrible mistake. They chose based on preference and had no way of predicting that valve would decide to release future best looking cosmetics to only fit the second style of the ti7 shoulders.
So since there are rumours that a kunkka arcana is coming with the battlepass, it's probably going to be green themed again, to exclusively match the second style of kunkka shoulders. It won't be blue because valve has already decided that green would be the better and "cooler" colour for kunkka's best cosmetics.
And all I want to say is this. If the Kunkka arcana is also green (and it most likely will be), give players who own the ti7 kunkka shoulders a second chance to unlock the green style.
Those who chose the blue style in 2017 had no way of predicting that they would be screwed for all future cosmetics because future cosmetics won't match the blue style.
Source: https://www.reddit.com/r/DotA2/comments/uzs0b5/if_the_rumoured_kunkka_arcana_is_released_to/