Identifying this chick's mother/Leg colour genetics

[Somewhere_In_The_Clouds]

I spent a completely unnecessary amount of time playing with the chicken calculator trying to get an idea about what possibilities my potentially mixed breed eggs I set would give me.
Then this chick hatched and threw all my hours of research and predictions out the window

Now, all the research (and the calculator) I did says this chick should not have yellow/orange legs!
The egg it hatched from is light coloured, not a Barnevelder. Before the chick hatched I would have confidently said it was a Sussex egg...
I think the father of this chick is a barnvelder (silver laced?) due to colour ( only other roosters are sussex) so Sussex X Barnevelder = White shanks...
Am I missing something here??
The two conclusions I've been able to think of is
1. The Sussex hens are hiding something (which for some wouldn't entirely shock me...)
2. I accidentally set my mix's egg and not a sussex...
Only thing with option 2. is Elly doesn't have a single comb (rose, I think).

 

[Amer]

The possibilities I can think of:
1. The legs will get lighter as it ages.
I looked up pictures of Sussex and Orpington chicks and while some had pink shanks, others had orange.
2. The Sussex are mixed.
3. Your mix has one copy of the single comb gene.

 

[nicalandia]

White Skin color is dominant over Yellow skin. It may hide unnoticed for generations

 

[Somewhere_In_The_Clouds]

1. The legs will get lighter as it ages.
I looked up pictures of Sussex and Orpington chicks and while some had pink shanks, others had orange.

I did think of this, currently the legs are the same colour as the Barnevelder chick (I think purebred at this point) that has also hatched.

2. The Sussex are mixed.

This is what I am leaning towards, I'll explain in a second.

White Skin color is dominant over Yellow skin. It may hide unnoticed for generations

I'll be annoyed if my sussex have it, but I think it's the sussex I bought last year...
I've remembered that I already suspected they were mixed with something, I kept them to just see and because I also needed layers. The group of pullets and cockerels I got had two yellow legged cockerels, one also had dominate white (neither of which I kept), and the eggs I got from the same person, I think hatched out one yellow legged pullet.
Also going off the size of the chick (quite a bit bigger then the Barnevelder) I think the Sussex are the most likely suspects at this point.

 

[Somewhere_In_The_Clouds]

Just updating at 4 weeks old to make sure I can definitely say yellow legs, as they have lightened a bit? I've already got the hens set up to test breed anyway.