Fresh kelp should be dark green, and some with thicker meat are brown. Generally, there are two kinds of kelp in the market: wet and fresh, and dried kelp. After soaking for a long time, kelp will delaminate and the epidermis will fall off, which should lead to the so-called fading.

if the kelp is only discolored on the surface, it may only be the result of a long soaking time.

another possibility is to dye kelp. Dyed kelp is mostly dyed with industrial raw materials, and the main raw materials for dyeing are "sodium dithionite" and "alkaline green". "Alkaline turquoise" is a chemical dye for clothing, and "sodium dithionite" is a stabilizer. Some unscrupulous traders also dye kelp with sulfur dioxide. Regular consumption of dyed kelp will cause precipitation in the body and may cause cancer.

Extended information:

In addition to being green in color, the dyed kelp has the following characteristics: because the dyed kelp has been rotten by chemical dyes, it is easy to be torn by hands. Under normal circumstances, the kelp skin is smooth and tough, and it is not easy to be broken.

The seafood flavor of fresh kelp is very strong, but the seafood flavor of dyed kelp is not obvious, and it has the odor of chemicals.

when consumers buy kelp, they'd better go to regular shopping malls and supermarkets, and don't bother. They should try to buy dried kelp and soak it at home, which is safer to eat.

Reference: CCTV-Consumers should be careful to buy green kelp

Baidu Encyclopedia-Kelp.