Juliet knows she shouldn't be seeing Romeo, but she says that the night/darkness will hide her. When she says mask, she is implying a mask that covers something up, like how the darkness will cover her up and hide her, so no one knows she is with Romeo.
Act 3. Scene 2. Line 18
For thou wilt lie upon the wings of night
Romeo is saying that Juliet is so light and good, like day, that when birds are around her they would mistake it for day, not night.
Act 3. Scene 2. Line 20
Romeo is showing the night to be secretive and mysterious, unlike the day, which is crystal clear, and has no secrets.
Act 3. Scene 3. Line 179
Or by the break of day disguised from hence
Romeo will rely on night to hide him to be unseen. The wings resemble hope for him to escape, as well as wings protecting him. Even though people don't have wings, she is personifying the night.
The night is almost taking care of Romeo and hiding him. Once again the author uses personification here. This also shows that the night won't be harsh and will lovingly protect him and keep him hidden
Come, gentle night; come, loving black-browned night
Romeo will have to disguise himself once it becomes day. He is safe and hidden in the night time, but not in the day time.