Sunlight is ultraviolet radiation. That radiation will fade the colors of the painting, no matter what medium the artist used. The sunlight can also discolor the paint.
Heat from direct sunlight will dry out the paper or canvas, and may even cause cracking. Heat will cause the paint to dry out and flake. If the painting contains any collage materials such as paper, they may fall off entirely.
Hanging a work of art in a sunny room where direct light never hits it, or in a place where sunlight is reflected onto it is harmful. This may slow the fading and drying process, but the damage will still occur.
Some people hang artificial art lamps over the artwork to spotlight it. Unfortunately, any type of lighting will cause damage to a painting. Artificial lights also emit heat and radiation that fades the color.
Putting the art behind glass, or installing dimmed windows to lessen the rays is also not effective. The radiation will pass through the glass, although the damage may take longer to occur.