The doors are painted like this because it is a federal law that all cabin doors on commercial airplanes should be outlined in a color which contrasts with the fuselage color. It is done so that in case of an emergency on ground, rescue crew can find the doors easily and open them quickly.
It happened in past years that passengers survived some crash landings (especially at night) but perished because the rescue crews could not locate the cabin doors in time to rescue them from the burning airplane. The contrasting outlines of the doors allow them to find the doors instantly.
Here is the actual text of this federal regulation:
14 CFR 25.811 (f) Emergency exit marking
(f) Each emergency exit that is required to be openable from the
outside, and its means of opening, must be marked on the outside of
the airplane. In addition, the following apply:
(1) The outside marking for each passenger emergency exit in the side
of the fuselage must include a 2-inch colored band outlining the exit.
(2) Each outside marking including the band, must have color contrast
to be readily distinguishable from the surrounding fuselage surface.
The contrast must be such that if the reflectance of the darker color
is 15 percent or less, the reflectance of the lighter color must be at
least 45 percent. “Reflectance” is the ratio of the luminous flux
reflected by a body to the luminous flux it receives. When the
reflectance of the darker color is greater than 15 percent, at least a
30-percent difference between its reflectance and the reflectance of
the lighter color must be provided.
(3) In the case of exits other than those in the side of the
fuselage, such as ventral or tailcone exits, the external means of
opening, including instructions if applicable, must be conspicuously
marked in red, or bright chrome yellow if the background color is such
that red is inconspicuous. When the opening means is located on only
one side of the fuselage, a conspicuous marking to that effect must be
provided on the other side.