You can use a colorless, water--based, acrylic paint as a sealant for the marker designs. Just brush a thin coat over your designs and allow to dry for 24 hrs. I'd probably turn jeans inside out and apply the acrylic to the back side of the designs as well, to prevent fading from both sides. Once dry, the treated fabric will feel stiffer than the rest but, it should soften up when washed.
If it is the design you love and not the color, draw over it with a bleach pen and you will have a white version. If you want both design and color, then I agree with previous posts, gentle washing and repeat as necessary. A sharpie marker should stay in fairly well.
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