Can Green Lantern Create Anything?

The Green Lantern ring can do anything so long as the wielder's willpower is strong enough. Most commonly, a Green Lantern Ring is used to shoot energy beams, fly, translate all languages, and of course, create green light energy constructs. With enough willpower and imagination, anything is possible.

Can Green Lantern create elements?

The only limitations a green lantern has is their own will and the ridiculously high level of will their rings possess. Will doesn't technically have a common state of matter, so yes they can infact create liquid constructs.

Can Green Lanterns make permanent constructs?

A regular, non-rainbow Green Lantern can only create constructs in the good old green color of willpower. But we've seen many times that GLs create their uniform with the ring, and most cases it has black and white colors, too, which they shouldn't be able to create.