Even if you absolutely love the shade of purple that's on your comforter, too much of a good thing can be overwhelming. If you matched the wall paint to the exact shade of purple that's on the comforter, the room may feel too dark, creating an overbearing vibe or even a sense of claustrophobia, especially if the room is small or if all walls were painted this same color.
If the purple comforter is dark, opt for a shade of lavender or gray that looks good next to the color of the comforter. Take a picture of the comforter with you to the paint store to help select the ideal color. Often the color swatches at the store will show which shades are within the same color family, which means they automatically look good together.
Check the comforter closely to see if it contains any other colors, such as within the fabric pattern or design work sewn in place. If not, look at the stitching on the seams and also the color of the other side of the comforter, since some comforters are reversible. Take a picture of these secondary colors and bring the picture to the paint store to match it up. A paint color that picks up on a minor color in the comforter will create a unified look in the room and will make the comforter stand out more.
If you decide on a paint color that's the same shade as the comforter, use the color for an accent wall rather than the entire room. Paint the other walls in a neutral shade that goes well with the comforter, such as a gray or silver, to match a dark purple. If your comforter is a light purple color similar to lavender or gray, a darker wall color is suitable for some walls. Again, pick a color that is in the same color family on paint swatches. For instance, an indigo paint used on a wall or two -- with a light orchid or lavender shade of purple for the other walls, as is on the comforter -- will create a unified vibe.
Another way to pick paint colors you like is to go to an online color selection site, such as colorblender.com. Use the sliders at this site to dial in your comforter's dominant purple tone, and the site will show you other colors that fit the color scheme. Playing around with the tool for a while will result in some interesting options, one of which is bound to show colors you enjoy. Take a picture or printout of the screen and have the paint shop match the color.