While Simon has told you about why ALL Fire Engines SHOULD better be yellow/green, not red, he did not tell you why all Airport Fire Trucks you have seen are yellow, and all normal Fire Trucks you have seen are red.
PBS's "Firehouse Primer" on the history of red fire engines:
Before firefighting was a paid profession, most communities were served by volunteer fire departments. These firemen didn't have much money to spend on upkeep, and at the time red was the least expensive color of paint. Red wasn't the only color used, however. Before it merged with the force in Manhattan, the Brooklyn Fire Department painted its apparatus a two-tone green to distinguish them from the red apparatus of the Metropolitan Fire Department. Today, there are still different colored fire engines, but red is the most common color out of tradition.
I don't know whether this is true or just an anecdote, but it's a nice story nevertheless.
While in the US, where fire fighting is a local issue, you will see yellow fire engines with some fire dept's (mostly in rural areas, I guess it's because you have long unlighted stretches of street there), in your native Brazil, the central government has decided to use red for all engines. This is the same in my native Germany, where the color mandated by law for fire trucks used on public streets has changed slightly over the years, from a dark red over a orangeish red to a retroreflecting orange-red. The Swiss, where fire fighting is a local issue as well, have mostly switched to yellow fire engines for visibility reasons. So I guess that you have only been to red-engine territory until now.
For airports, it's a different issue: Unless ordered otherwise, Oshkosh's (the US market leader) airport fire engine lineup is painted green/yellow. Rosenbauer (big player in Europe) on the other hand has a red lineup. While you can order other colors, most customers stick to the default. So my best guess is that you haven't attended any plane evacuations in Europe until now.