It's sometimes hard to tell guppy genders apart. I was able to tell what I had when my guppies were 4 weeks. Males anal fins are more pointed, even if it's just a little, though it seems like every day it becomes a bit more pointed.
Now, at almost 5 weeks, my guppies are just starting to get color. Of course, their gray and black came in around 3 weeks. With this, it's just a matter of time. Males tend to get their colors before females.