It's generally not recommended to stain a deck if rain is expected within 24-48 hours, as it can wash away the stain before it has a chance to properly cure.
Yes, deck stain can change color slightly as it dries, so it's important to test the stain on a small, inconspicuous area before applying it to the entire deck.
The best time of day to stain a deck is typically in the morning or late afternoon when temperatures are moderate and the deck is not in direct sunlight.