According to Mythbusters, a white tinder doesn't work very well when trying to start a fire focused solar rays. The white color reflects too much of the energy. When they rubbed a little dirt on it it worked much better.
WARNING & DISCLAIMER:
SELECT AND USE OUTDOORS AND SURVIVAL EQUIPMENT, SUPPLIES AND TECHNIQUES AT YOUR OWN RISK. Information posted
on this forum is not reviewed for accuracy and may not be reliable, use at your own risk. Please
review the full WARNING & DISCLAIMER about information on this
site.