You can only pour a colour onto the same colour, or into a completely empty tube. That restriction is what makes it a puzzle rather than simple rearranging.

No. Pour & Reveal has no timer, so you can plan each pour for as long as you like. It is designed as a relaxing puzzle rather than a race.

Usually because empty tubes were spent too early. An empty tube is the only place an awkward colour can go, so pour into one only when you can see how it will be emptied again. Also check what a pour uncovers before making it.

Each completed tube reveals part of a hidden picture, so finishing a level uncovers the whole artwork alongside solving the puzzle.