This is another random observation I have noticed on Airplanes. The wings have two different styles of colouring (in general). They either have the center part painted in a metallic grey color or the wing is all beige/white colored. It seems that certain carriers paint all of their wings in one way or another for the most part. For instance, UA has all white on their wings while NW has all grey coloring. I was told a long time ago it had to do with anti-icing on the wing, if that were the case, why would some carriers do this and others don't when they both fly into winter climates and encounter icing at cruise altitude.
Here are pics of the grey/metallic seen on an A319 and a 757.
Here is the all beige/white seen on an A319 and a 757.