For those with significantly different areas of color on their face (including those with conditions like vitiligo, hormonal hyperpigmentation and severe sun damage) the question becomes: which area do I match?
The answer: For the most flattering match, choose the darkest shade on your face and match your foundation to that. Though this may mean some work blending the shade down onto the neck, it’s really the best way to avoid casting an ashy or grey cast to your skin, which is what will happen if you put a lighter shade over a large dark area.
Choose a foundation that offers medium to full, long wearing coverage. Popular choices include Dermablend and Kryolan Dermacolor, I've also been a fan of Estee Lauder Maximum Cover (but haven't tried it since the formula changed last year). Apply foundation first, then touch up areas that need extra coverage with concealer. If your discoloration is the result of bruising, port-wine stains, or rosacia, corrective color techniques can be your best friend- apply corrective color first, followed by foundation and concealer.
When you've gotten the desired coverage, top your foundation and concealer off with powder to set it. You can also use a sealing spray like Mehron Barrier Spray to help keep your makeup budge-proof all day long!