<p>I am doing a quad-copter now.</p>

<p>I am not sure what color is the best to use for outdoor durability, especially in sunlight.</p>

<p>They say PETg is the most durable material amongst cheap ones.</p>

<p>But what color to choose: white or transparent? Or it does not matter?</p>

Unless you plan to use your copter outside Earth atmosphere or expected lifetime is more than tenths of years, UV degradation should not be a problem for PETG. Some of the net sources indicate the
possibility
of degradation after significant time of
constant
exposure to outdoor conditions (mostly color change), which sounds mostly like absence of practical experience.
Yet if there are any concerns about losing mechanical properties from the sunlight, any opaque lacquer car paint can help to absorb unwanted radiation and add some nicer view to the model. Acrylic paint will also work (tried this once myself) but it may not be suitable for outdoor.