Thank you for your interesting question.
How Stuff Works - How Magna Doodle Works by Craig C. Freudenrich, Ph.D.
You'll see a Table of Contents with a menu. You can click on any of
the titles and be taken to that page, or else click on NEXT PAGE at
the bottom of each page. Pages include: Introduction to How a Magna
Doodle Works, Inside a Magna Doodle, The Heart of It All, Magnetic
Pen, Magnetic Eraser, The Case, In Action.
On this page: The Heart of It All explains the colors...
* The magnetic particles are dark, small and fine (about 10 microns)
- This allows them to be seen easily, to stack up and to be nearly
unaffected by gravity.
- magnetic oxides - black magnetite, gamma-hematite, chromium oxide, ferrite
- magnetic alloys - iron, cobalt, nickel
* Dye is mixed in - This provides high contrast to the dark magnetic particles.
- white pigment - most effective (concentrations of 3 to 10
percent) and most commonly used
- colored pigments - in some Magna Doodles
You might also be interested in the Etch A Sketch.
how do magnetic doodle boards work
Magna Doodle How Stuff Works