Stevenson,
It can be a function of strain and with some strains differences can be consistent between sexes. Females tend to be darker with smaller, more sharply defined "white" spot on head. Male white spot larger and more diffuse.
Also look at feet. Males tend to have lighter colored feet that are also slightly larger. This character is even more a a function of strain.
What I like to do is score each bird for all three characters: white spot, leg color, and leg size. If characters 2/3 or more for a given sex, then chick assigned that sex. Still not perfect but increases odds sex "estimate" is accurate.
Waite a couple more days and you can look at coloration of flight feathers. Feathers of males about 2/3 light grey while 1/2 light grey in females. This system error prone, especially with birds derived from Ideal Hatchery stock.
With some strains differences also in flight feathers of wings. Males have noticeable slower maturing feathers. Some commercial hatcheries have crossed in barred Plymouth rock to aid in sex determination. I am now removing these birds because they also produce darker eggs than typical of dominiques.