Color Wonder

Color Wonder is a marker pen and paper system made by Crayola, primarily intended for use by younger children, in which the special clear-ink marker only appears on the Color Wonder paper. The Color Wonder products debuted in 1993. Color Wonder paints and fingerpaints, as well as Color Wonder coloring books of popular characters such as Disney Pixar's Cars and Disney Princess also exist.

The 'magic' clear-ink products were designed so that toddlers and young children don't stain their clothes, paint on the walls, etc. Crayola has a patent under Binney & Smith relating to this kind of mess-free marking system. An applicator, such as a felt pen, uses a composition containing a clear leuco dye that is developed by color-triggering compounds in the paper substrate.

There is also another line similar to this one by Crayola, but for older kids called Color Explosion. It is like Color Wonder, except there is not only one color hidden in the paper, but stripes, dots, and swirls of a variety of colors, depending on the package you get (Fire & Ice, Twisted Tropicals, etc.) It is also available in black and white paper.

References

Picture of system
Product Page
Patent [1]