After 4 days fertile eggs should have changed in shape. At least they should have straighten and flattened like on this picture:
If they collapse (meaning the inside lookes smashed and murky) they may not be fertilized as you already said. I have seen this on some first batches of young females (independent of color) - maybe some kind of inexperience to allocate the sperm.
On the other hand sometimes (but rather seldom) some Axolotls do not fit together (also independent of color as far as I know) meaning there is a high rate of unfertilized eggs or dying embryos. I do not know the reason for this but I suppose some genetic incompatibility.
As for your eggs: Just have a look on them, maybe some of them will develop. And if not, give the Axolotls some time, I am sure that there will be another batch.