They could. I've had quite a lot of chicks hatch out on day 19 when I incubate.
A weird colored dot on the egg could be a sort of bruise, which isn't the greatest sign because it likely means the chick broke a blood vessel inside the egg; I've never had any die from that, but others have.
It could also possibly be a spot of what I assume to be rot; when I was a kid, my family had ducks and they'd nest wherever they liked around the yard. Then, after some ducklings had hatched out, they'd just leave the remaining eggs for dead. Many times we would go out there to find that one or more of those eggs were still alive, so we would bring them in and hatch them. Sometimes they'd have odd spots on them, which seemed to be isolated areas of rot -they were odd greenish slime once the chick was out and we could see- but they didn't seem to harm the chicks much. We even had one egg/duckling with maggots on it once; we gave it an immediate bath and it was okay after that.
I can't think of any other options for what a newly strange colored spot on an egg could be.