I'll try and get one as soon as possible it's quite late where I'm at but I'm thinking right around the area marked in the picture. Don't know if that helps much but thanks for trying to solve this mystery!
They are about 7 weeks but I think this picture is from when they were a bit younger don't have one of them currently. But right around where a comb should be is where they are getting red.
My grandma recently bought two little ducklings and they have been growing faster then weeds! But we have a yellow one that doesn't walk much even when outside. She hobbles for a bit and then will just lay down.. In the water she loves to swim but walking around doesn't seem to be her strong...
From my research the bottom one is a definite roo, but the top one looks more like a hen. Don't count me on this because as you can see I'm needing help with my little one as well.