Lt Brahmas and Light Sussex are based on 2 different genetic alleles. The allele is the basic genetic canvas upon which the colors are painted. The Light Brahma s based on the eb (Brown) allele. This means the underfluff has a hue of color to it. The underfluff must also be properly balanced in color to achieve the proper top color in the bird's plumage.This is why it is possible to see black feathers scattered among the white feathers on the Light Brahma's body.
Conversely the Light Sussex is based on the eWh (Wheaten) allele. This means the underfluff is white to the skin. There is no color balancing needed to get a white body on the Light Sussex. The only time one should see any grey or black feathers in the white areas of a Light Sussex is if the breeder has concentrated too much black in the hackle and the color bleeds out onto the top of the back from the hackle. For my money, the Light Sussex is a much easier bird to breed for color which eaves the breeder more energy to expend on breeding for breed type.
Incidentally the production virtues of the Light Sussex are so closely allied with the breed type that literally..if the bird doesn't "look" like a Light Sussex... it literally isn't a Light Sussex. A great breed variety. If the weather here hadn't turned so bitter in the winter, I would still have them.