My guess is you got not very well-bred Cuckoos. Are the hens laying yet? If so, what is the egg color like? You can have yellow legs in hatchery lines or from flocks where, well, not a lot of thought is put into breeding birds to any kind of standard. This kind of breeding happens a lot--there is such a demand for the birds there's just a lot of breeding going on because everyone wants dark egg layers--problem is, when you breed willy-nilly like that, egg color suffers, too.
The correct color for the leggs is that flesh-colored tone. I believe there may be a little leeway there (gray-ish tones are ok, too, I think?) but any yellow is a fault.
Also, you said you got 8 roosters? Out of how many total chicks? Just curious, because Cuckoo chicks can be sexed at hatch with pretty decent accuracy. If you got 8 roosters out of, say, 10 chicks, someone wasn't quite on the up-and-up with you. Now if you got 8 out of 20 or 25, well that's good!
Regardless, you have some pretty birds and hopefully you got birds that are suited to the purpose you wanted them for.
I wouldn't recommend breeding them, though, if you wanted to start a Marans breeding program. If all of those birds came out of the same flock, even the ones with the correct colored legs are probably carrying the yellow-legged gene--and it can be really hard to get rid of. You'd spend a lot more time and money trying to breed out the yellow legs and any other faults in type, as well as improve egg color, than if you started out with better quality stock from the get-go.