Wood storage doors are pretty, but can rot and break easily, and evidently display signs of weather. Steel can rust and dents very easily. Aluminum is lightweight, but a reasonably unattractive, un-paintable surface. Vinyl is a positive insulator, but looks fake-y.
Composites don't have a tendency to take up drinks as naturally as wood does (a element of what makes them so hearty!) but this capacity that staining can be barely of a fight without the wooden grain. Be sure to test the stain on a small, out-of-the-way subject first to work out if the colour results are to your liking. And if you have a faux-woodgrain pattern in your door, it should still turn into looking quite nice.
And if you're trying to go the paint route, once again you have to prime the surface of the door this time with an opaque exterior latex paint. The a laugh thing about this stuff is that it's like gesso for artists- you can use it to smooth a surface, or add texture with certain brushstrokes. It can extremely wake up the inner artist in you! And then of course you get to add any colour(s) you need on top.
However, there is something you have to take into attention with a composite drapery door, and that's if you need to colour, stain, or varnish your door. You have to keep certain things in intellect, but it doesn't absolutely preclude you from doing it! It only takes barely of more planning than, say, a wooden door.
Once you're done with any (or all!) of the above strategies, be sure to add a UV-resistant sealer as a top coat to maintain the integrity of your surprising new piece of home art.
For a varnish, you have to test the properties of the product you will be using. Again, do a conference run, and remember that the foundation of the composite (whether it be plain white, oof-white, gray, or another colour) may impact the resulting hue once the varnish dries. If you need to make sure this doesn't appear, you can apply a primer layer of what's known as stain sealer to the surface of your door, ensuring that the colour will come out as you need with little or no interference.