HI! First of all
Are these your first venture in to chicken ownership and are there more or just these two?
Guessing about roo or hen can be tricky and sometimes guesses are wrong, especially since we are only seeing little bitty photos.
There are some generalities to go by that guide the thought process.
-In general roos feather out slower than hens--especially on the wing bows (sort of above the wrist-bend on the wing), back, chest and tail but not always.
-roos combs and wattles pink up and grow faster than the hens of the same breed. Now a large-combed breed like a leghorn can have a really large comb at 4 weeks on a hen compared to a small combed variety, so you have to be aware of the breeds. Pea combs can be really hard to sex early on.
-Little cocks tend to stand more erect and alert-looking than little pullets
-If you pick them up and hold them so they are on their backs in your hand, the roo tends to struggle more and resist whereas the hens relax and accept it more quickly
-I think the roos are more likely to be the first to perch--they seem to like to be high and watchful/safe
-The roos will start to crow, maybe at 4 weeks, maybe much, much later. It may sound like a squeek or it may sound like someone is sobbing or getting strangled.
If possible, it would be great to get better close-up side shots of the chick and a heat shot straight on to see the comb better. I can't say for sure on sex, but the brown one seems to have a larger comb which is suspicios for roo, but I think its too early to say. My first gut was it was a Speckled Sussex, but I would need more pictures to give you a meaningful guess. Not at all sure on the other and definitely need more pictures.