If you use the blue or purple markers to mark your quilting design, be sure not to let the quilt sit in sunlight. Heat can permanently set those markers.
Some quilters "spritz" the markers away, but when you do that they can reappear at a later date. It's safer to rinse the entire quilt thoroughly. If you do this in a washing machine, it's a good idea to run a rinse cycle through the machine first to make sure the water will be entirely clear when you rinse the quilt to get rid of the marker.
Also, never let those marks stay in the quilt for years (in storage, for example) as there is too much risk of them getting heatset at some point.