I don't have the solution only bad news. I don't recommend ever ever ever ever using yellow The mfg claim is not true that it will come off. I tested some from one mfg a number of years ago and I could not even bleach it out
Now having said that. My onky suggestion is first contact the company and tel them what the package says and the results.
Then try with very soft toothbrush and something like Shout or other stain remover or one of the stain removing sticks.
See if you can repeat the process on another fabri before trying on your quilt.
My devilish nature might say to rip it out and requilt it with yellow thread......but then that wouldn't be nice.
Another recommendation is to brush or erase any chalk marks before washing. Chalk will cake and the more you remove the easier it will be for the wash to do it's thing.