Because you have two to compare to each other, the most obvious sign is the difference in their wattles, those two sets of red fleshy things hanging under the jaw. I apologize for the non-scientific description but that's the easiest way to describe it if you don't know what 'wattles' are.
In any case, the light colored one has two outstanding sets while the darker chick has almost nothing to see under her jaw. In my experience males always develop those wattles much earlier than females and they are always much more pronounced. Most of my chicks have been mixed breed so for me the easiest way to sort male from female chicks is those jaw wattles. It gives the guys away every time!