The reason for this, I think, is that I am a premium user. I can return home from any tile on the explore island, even the green portal.

You see, the portal button does not unbind from the purple to the green when I step onto it. I've tried pressing the button while standing on the green portal, but I only exit the island. The green portal is supposed to put you on another island, no? Or is it supposed to take me away from the exploration?

Is there a way of entering the portal I'm not aware of?

Please check the recent news post for the details on the expanded explore system.

You need to have a Portal Energy Cell in your inventory to use the green portal. You can't activate the portal if you don't have any cells, and the portal button remains a normal "exit island instantly" button. If you have at least one cell, it turns into a button that allows you to move to the next explore level.

You can't exit the explore from the green portal tile If you have a Portal Energy Cell.

Nevermind! Nightingale cleared it right up for me, haha!
I'm just being silly as usual~ I have to get myself some Portal Energy Cells! Sorry, I guess the sheer amount of text in the news post simply confused me XD

EDIT: Oh gosh, I was even sniped by Angel! XD I'm sorry, I remembered about the cells as soon as Nightingale mentioned them. I must have been too excited about the Lykos and things~ I will make sure to be more thorough in the future.