The lighter colored duckling could be a welsh harlequin. Depending on how old it is, it could be drake (they are a sex-linked breed, males have dark bills and females have light bills but this rule only applies for the first few days after hatching).
The darker one looks like a rouen.
its hard to tell breeds as ducklings and u will tell sex when the start talking rather than chippin as sexing is only taken place as dayold and is extremely dificult u would need expert so listen for the diference in voice like amiga says i find as they get older the boys get quiter most of the time and the girls get louder and noisier. send pics of them as older ducks and looks like theres lots of people on here that could tell u their breed