If you haven't prewashed and the quilt runs, don't panic....try soaking it in Oxyclean for several hours or overnight and it will usually come out and the colors stay bright. I have a friend who occasionally bring me a quilt top or blocks that she picks up at estate sales. She recently brought me the cutest Sunbonnet Sues which were appliqued and the stem of the flower she was holding was hand-embroideried in green. One of the blocks had some stains on it, so she washed it by hand. The green embroidery thread ran! She said to just put it in the corner where it wouldn't show much! I used an Oxyclean paste and a Q-Tip and was able to remove the stain. She didn't want the quilt washed before I gave it to her, so I pinned a couple of color catchers to the quilt with instructions to use it on the first wash.
I pre-wash most of my fabric, but I also use the hot water in the cup test if it is something I would rather not pre-wash before cutting. Occasionally patterns will actually tell you NOT to pre-wash...OBW, etc.