I havent noticed any difference with the eggs but I dont have coffee grounds in the nesting boxes. Only on the poop board. My babies have lighter feet and I havent noticed any discoloration but again, I only use them on the poop boards.
We just recently switched to coffee grounds also. I've only had them in there for about a week but love love love it so far we are replacing what's lost with our own used coffee and I'm hoping our coffee addiction will help keep it replenished.