How the color yellow is used throughout breaking bad
A lot of people have said that the yellow tint they give during drug deals means they're in Mexico, but notice how this tint appears even when they are in Albuquerque. It's used really only when the characters are very deep into the nastiness of this dark drug world. The yellow is used throughout the show to mark the criminals.
Gus always wears yellow around his restaurant, but not many other places. Like Saul he can hide behind his business and civic involvement, thus the tie covering his yellow shirt.
As Gus' restaurant is an integral part of his operation and the forward-facing component, it's naturally yellow. As some of his employees (such as the female manager) are slightly involved it makes sense that they wear yellow uniforms as well.The yellow sign is a pretty clear symbol of how Gus hides in plain sight.
Notice how you don't see yellow around the laundry much, except in the temporary suits they wear when they're cooking (neither Walt nor Jesse wear yellow often during the period they're working in the lab).
In the early seasons Jesse wore yellow frequently, at a point where his life was based entirely around an amateur drug business (which he advertised in his license plate and website). Since working in the lab and trying to hid this life from public view, yellow is only a color he puts on when he cooks.
And of course it's the color he dons when it's taking care of dirty business, solving problems he wouldn't have without the drug world.
And this came after Crazy 8 planned to stab Walt with a yellow piece of broken plate (thanks wildebeestsandangels)
Additionally, Gale's death was as he was briefly coming back into this world as a lab assistant, and as the drug business is what killed him, it's only appropriate he die in a yellow shirt.
Saul isn't always directly involved in all of Walt's and Jesse's operations, and can shield himself from the drug world with his law business (thus the yellow being hidden under his suit). He only sparingly wears yellow, though.
After the lab is destroyed, Walt and Jessee hide their illicit business inside another business with yellow house covers
When Walt starts bringing his business more to Skyler's attention to the point of her moving the children out, Walt starts to wear yellow more at home, since he is bringing this criminal world into his home. He wears this shirt during the 2 minutes of prison murders.
Drug kingpins also wear yellow. Tuco is an exception to this, as are the cousins and Tio Salamanca, though most of them play the role of enforcer, as Mike does. Foot solders don't wear it.
And of course it all started with a little yellow mustard stain on Walt's diagnosing doctor (thanks riggerdigger)