Nope, you cannot color-sex millie chicks. The lighter chicks become the darker 'teenagers', strangely enough. But by adulthood they are nice birds!You have two different coloured chicks, are the light ones male and the dark ones female? I had two eggs hatch out with one of each but the dark one didn't make it and the light one has suddenly become a boy. Just wondering.
That's why I tell everyone to wait as long as possible to decide which birds to keep or rehome. You could give away your best bird just because s/he is going through a fugly stage.