About your comment "and whatever the genes are behind what you see" does that mean that for example a silver columbium chicken that is Light Sussex type but genetically e^b based is still a Light Sussex?
No. The Light Sussex is eWh/eWh S/S Co/Co. This is very important. Being eb ( Brown) based means the undercolor needs to be balanced correctly in order to get correct top color. if it is not, there will be black stippling in the white feathers in an eb based Silver Columbian.
By contrast the Light Sussex is eWh ( Wheaten based). This means the undercolor is white to the skin and needs no color balancing of the undercolor to get correct top color. The only reasons one would see black or grey in the white parts of a Light Sussex would be if
1. there were extra genes or modifiers,
2. an incorrect allele base ( as in eWh/eb or eb/eb),
3.or if the breeder had concentrated too much black in the hackle. Too much black in the hackle can cause the bottom of the hackle to spread out in the manner of scattered partly light grey feathers on the birds' withers.
In regards to number 3 above, I have found this can be seen in the chick down. The proper eWh/eWh Light Sussex chick should be light yellow all over. I had a hatch where some of the chicks had a tiny slightly darker "tinted" down at the nape of their necks. They feathered out correctly. I had a subsequent hatch where the nape of the neck was just a bit darker yellow than that 1st hatch. Let us call it a "hue" rather than a "tint" of darker yellow. This second hatch of darker hued yellow napes feathered out with the black hackle continuing across the withers and coming to a point,...this area beyond the proper hackle area consisting of light grey tipped feathers scattered among the white feathers.
Point is, one can see this in the chick down and cull it out. If the down at the nape of the neck is just a "tint" darker, it is warning blood that too much black is being bred into the hackle but the bird will still feather out a proper hackle.
If the down on the nape of the neck is that slightly darker "hue" it is warning that one has crossed the warning blood threshold and too much black is in the hackle, causing it to polka dot the withers with 1/2 light gray feathers.
Best Regards,
Karen in western Pennsylvania, USA