I used paste dye that I had from a cake decorating class. They have it at Walmart and hobby lobby, in the cake decorating areas. I applied it to the vent with cotton swabs, but will have to do it again. Only one laid that showed any color. Instead of on the vent, I'm going to have to try IN the vent, although my children say,"won't they just poop it out?. I may have to use lipstick. It may stay on better. Going to go check nests right now. We haven't been home most of the day.
Added: nine eggs, no dye color. Yesterday had proof of one layer with black dye.
Cut a sliver of the cheapest lipstick you can get at the dollar store, and insert it in the vent. The cheap stuff tends to melt and smear better. If your getting most your eggs about mid-day, lipstick them in the morning. If they are laying in afternoon, lipstick or food color them about noon. For me a good squirting of food coloring works best.