Hi everyone, long story short, I bought 6 eggs from someone who has 'pure bred khaki campbells'. 3 hatched. But one came out yellow!

We'll love it all the same, but I am wondering how this could of happened. The breeder only has 4 khaki's, no other ducks at all to get mixed up with, he is just as surprised as me. I'll post a pic of the parent's. Could it be some kind of throw back? Or mutation? I can see a tiny bit of white on a parent drakes neck?
Could this surprise yellow khaki campbell get some light brown speckles through its adult feathers?
I'll keep you updated with pics.