The only way to truly kill mildew is with bleach or hydrogen peroxide.
Both have a tendency to lighten colors.
For painted, colored surfaces (such as your door), experts recommend a
weak solution of bleach. Test an inconspicuous spot first; treat it,
rinse it, and dry it, and see if any lightening occurs. If it does,
use a weaker solution.
Nonetheless, Virginia Tech recommends a solution of 3/4 cup bleach per
gallon of water as a good solution for painted surfaces. Use an old
sponge that you will throw away after use. Treat the surface of your
door, then rinse thoroughly with water and pat the area dry. ("How to
Prevent and Remove Mildew In Relation To The House and Household
Furnishings," Virginia Tech:
Virginia Tech also recommends commercial cleaners for painted
surfaces. Many experts recommend H2Orange2, which contains orange peel
oil, detergent, and hydrogen peroxide. It's the hydrogen peroxide that
will kill mildew--but might also cause some mild lightening. Be sure
to test in an inconspicuous spot. ("Remove Mildew," Do It
Yourself.com: http://doityourself.com/clean/moldmildew.htm )
When you do eventually repaint your door, be sure that you don't use
an Alkyd oil-based primer or paint. Such paints have organic materials
that attract and promote mildew ("Painting to Preserve, Protect, and
Beautiful Exterior Wood:
http://alsnetbiz.com/homeimprovement/faq6.html ) In addition, before
repainting, you'll need to kill all the mildew with a strong bleach
solution. Finally, prime using a mildew resistant primer.
cleaning mildew oil paint