Color or Black and White


I used Pentel Fabric crayons to color the first bird.
When I used the iron to set the color, the blue pen marking reappeared.
I used a pencil to lightly sketch the second bird.
Maybe if I had a definite project in mind, I could decide whether to use crayons or not.
I do not know how permanent these crayons are, even though the directions state that setting with heat will make color permanent.
If any of you who have worked with fabric crayons could share your experience, it would be greatly appreciated.
Also, does anyone have any recommendations as to what type of pens can be used for sketching without leaving a marking?