Getting out purple marking pen?

I was doing some hand embroidery on white fabric over a couple of days. The disappearing purple marking pen I used is one I use all the time. it always washes out with water or just disappears. Well I had to go back over a couple of spots because it took me so long to finish the embroidery. Now there are several dots (which have turned gray) that I can't get out. I hand washed it twice, once with clear water, once with mild detergent. I'm waiting on it to dry, but they're still slightly visible.

Anyone know anything foolproof that gets it out?

I love Fels Naptha soap which is a hard bar soap which can found at most grocery stores.  I lightly rub it onto stains and hand wash.  It works! 
BUT, of course, I would always test it on a little patch of the fabrics in your project first!