Wearing armor causes hotbar to become opaque.

[Joni-Origami]

Wearing armour (on hypixel at least) causes the hotbar to become opaque, and removing the armour causes it to become it's normal transparent self.
aswell as this, sometimes when taking armour off, the hotbar will become red for some reason.

[Lugent]

Can confirm, in this case, a leather armor makes both hotbar and experience bar to become brown tinted. (Plus fully opaque too)