question about purple fabric

I have a question. I did a google search and can not find my answer.

I have noticed that when I iron some of my purple pieces, the fabric will
change color. Most of the time, the purple comes back, except yesterday
ironing a Debbie Mumm piece, the color did not come back. Why does this
happen and should I use these pieces in my quilt or should I discard these
pieces as inferior?

Thanks!

Sandi M.