I can't speak for the museum but I would guess that the subdued lighting is due to the fact that colors show to their best advantage in that light. Bright artificial light will flatten out colors, make them appear dull, lack luster and even fainter than they truly are. The amount of distortion of color refraction depends on the amount of lumins, the color of the walls (how much reflection is going on), and other factors. Subdued lighting is common in all museums. Special high lighting is usually done with spot lights in the ceilings and these are designed so that there is no reflections or glare and they themselves are soft lights. But if artificial lights would fade, there would be no spot lights on paintings, quilts, carpets, tapestries, etc.