Whether one is displaying a 500 year old masterpiece at a museum, or a contemporary image printed using modern methods, all art pieces are sensitive to ultraviolet light. Ultraviolet light is a very damaging light wave, mostly found in natural sunlight, but also found in smaller quantities in household lighting.
Your portrait should be framed by a trained expert who understand the archival materials and handling of your portrait. All watercolor prints should be framed behind glass, using acid free materials.
Your portrait will look its best if it has an appropriately color balanced, UV protected light. Consult with your artist about their recommendations for the highest quality product available on the market. Your portrait painting should not be exposed to direct sunlight, as it will likely damage it in a noticeable way within 3-5 years.