Is this a Buff or something else...?

[Senna95]

You 2 "buffs" are males. Not sure about the runners. Bill color has nothing to do with their sex.

 

[Cluckcluck1215]

You 2 "buffs" are males. Not sure about the runners. Bill color has nothing to do with their sex.

Not to be harsh, But on Runners it does.
My Fawn and White male I used to have had a yellow bill, And my female I have has a dark green beak.

 

[Senna95]

Some colors do have variation in bill color between the sexes (ie. harlequin), but I'm not sure if fawn/white is one of them. Sometimes the bills change color after hens start laying due to nutritional and hormonal changes. Either way, it would be very difficult to sex by bill color until your ducks are in full adult plumage, since bill colors will change along with the feathers. The easiest way to tell at a young age is by voice sexing. I can tell by 4-6 weeks of age this way.