Yes, indirect sunlight will fade fabrics. Actually, any kind of light will fade fabrics (including flourescent lighting). Dyed fabric has a finite number of light-hours it can tolerate before fading takes place. This varies with the color and dye type, but often is in the neighborhood of 45 hours or so of direct sunlight. It takes longer for the fading to happen in indirect light or artificial light, but it still will take place.
There is a protective spray you can get. Might be worth treating a quilt that is hung, to delay the fading. Sorry, I can't remember what it is called. It's the same kind of treatment used on curtains and furniture fabrics to retard fading. I think it deflects some of the damaging wave lengths.