It can still be Shanks which means Oda is playing 4D chess lmao. Idk what's going on in this manga anymore. Regarding his hair, during the talk with Gorosei we can only see bangs and Shamrock has the same bangs.
Some people say it can still be Shanks cause 1) Shamrock wouldn't wear a cloak to hide in Mari Geoise and 2) Gorosei talk to him kinda strangely...But:
It's not like this cloak hides his face, you know, everyone still can see it's him (or Shanks). Shamrock and Gunko are wearing the same cloaks, but it seems they don't care about being noticed and revealing their identities that much considering they came with the lightning strike, beat up the guards and make Loki scream. So I thought maybe the cloak is just a part of Holy Knights regular look? But Garling never wears it. I feel like it's usless to make it make sence cause this cloak's main purpose is to give character a mysterious look.
Another argument is that it's weird that Gorosei tell him that politics are not for him, but we actually don't know how much HKs are usually involved in politics and we don't know if the second in command of HKs (which I assume Shamrock was at the moment of their talk) is supposed to involve himself into politics. Maybe they just execute special orders from Gorosei and watch after other CDs behaving, maybe not. They also talk about him "coming all the way here" tho, this one I can't explain, maybe Shamrock was away from Mari Geoise.
There's also a chance that it was supposed to be Shanks first, but Oda chickened out and created Shamrock.