I bought salmon a week ago and wanted to make it for dinner today. When I took it out of the refrigerator, it had a yellow stain on it. What is it?

The most common cause of a yellow or pale discoloration on salmon is fat bloom.

What is fat bloom?

Salmon has natural fat.

When stored in the fridge, especially for several days, the fat can rise to the surface and oxidize.

This oxidation can create yellow or pale streaks or patches.

Does it mean the salmon is bad?

Usually, no.