Muslin is woven loosely, with small gaps between the threads that let air move through and let the fabric dry fast after a wash. That open weave is also why muslin gets softer with every cycle instead of fading out. Color loss over time in cheaper fabrics usually comes down to the dye and the finish rather than the weave itself, which is part of why we compare muslin cotton against regular cotton for babies when parents ask about durability.
We dye our double-layer muslin with child-safe dyes and pre-wash every piece before it leaves Bali, so the color you see on arrival is already stable rather than sitting on top of stiff, unwashed cotton. To keep it that way, wash cold on a gentle cycle, skip fabric softener, and either tumble dry low or hang the piece to dry. High heat and softener are the two things most likely to dull color and roughen the fabric over time, more than the number of washes itself.
Some fading over months of regular wear is normal for any cotton, muslin included, and a little creasing after the wash is normal too, not a sign of damage. If you're choosing between styles for a newborn, comparing a muslin cotton onesie against a muslin cotton romper is more about fit than fabric care, since both hold color the same way as your baby grows through the newborn to 4 year range we size for. Either way, the fabric keeps softening long after the color has settled.