what does it mean that Romeo sees "crimson in [Juliet's] lips in [her] cheeks, /And death's pale flag is not advanced there"?

Pretty much that Juliet is still alive (she only sleeps) but Romeo doesn't clue into this, he still thinks she is dead.

I don't think you are correct there Aslan.

Shakespeare has Romeo looking at her face and saying that even though she is dead, she is still beautiful. Saying, using the flag metaphor, she still has the red flag of life and death's white flag, although it has coloured some of her face, it has not coloured (conquered) all of it. The red flag. the colour of life, the colour of blood, is still metaphorically raised, and the pale flag has not yet, been raised over Juliet.

He is in awe of how one can be so beautiful in death. He sees that there is colour in both her cheeks and lips and questions why it has not yet drained from her body. Romeo however thinks that it is because she is so beautiful even in death she is radiant. He doesn't think that Juliet looks dead. This is dramatic irony (when the audience knows something that a character doesn't) because the audience know that Juliet is in fact just under the influence of a sleeping potion but Romeo believes her to be dead.

the colour of white symbolises a lack of something. Most commonly a lack of knowledge. This is key in this quote as it shows Romeo's lack of knowledge as he is oblivious to the fact that Juliet is dead