Now that 5 out of 7 girls have been laying for about a month, I have the answer and more! I can tell the girl by her egg and yes, colors from egg to egg (from the same hen) cany vary slightly. Matilda, a black Australorp, started out with heavily speckled eggs. The speckles were large and plentyful and now they are barely there. Her eggs are the darkest. Her counterpart, Clemetine also speckled her eggs, but in her case the speckles are white! The Wyandottes lay beige and very chalky looking eggs and Polly (BR) lays a shiny light brown egg.
And best of all, the eggs are homegrown and delicious!