Hi,
I've been coloring quilts with crayons for over 50 years. Many of them were baby quilts washed all the time. The first ones I made have been handed down to the next generation and they still look as look today as they did 50 years ago. The material is wearing out but NOT the coloring. I didn't know to use white crayons as an underlay - I never used it but I don't think it needs it. By the way no special laundry soap was used on them - they were gathered up with the sheets and dumped into the washer and whatever laundry soap was on hand was used.