I am looking for a wide range of non-toxic, washable colorants that
are VERY concentrated colors in a "normal" state (like food color),
but turn clear after being exposed to a few sec/min of air/light/or
other natural element.

like with new Dulux wall paint ?
You paint with pink color and after it's dry it becomes white colour :)
cool

Take a look at the components of the kid's toy "Disappearing Ink".
Now, I don't know how you'll get more than a few variations on the
color (blue) as it's a chemical process, but I think it might be a
place to start.

http://www.uwm.edu/~tholme/fun/disink.htm

Litmus paper is a similar concept. 

Hope that helps.

I'm a science teacher and when I read your questions I immediately
thought of a pH lab that can be done with red Cabbage Juice as an
indicator.  You can quite a range of vibrant colors by altering the
acidity or alkalinity of your stubstance, for example very basic
solutions like bleach make a deep yellow while very acidic solutions
like vinegar make a magenta color when mixed with the Cabbage Juice
indicator.  To get the colors to disappear completely will be hard -
but I think it may work if you add a buffer (like alka seltzer) to
weak solutions.  The neutral color is a shade of purple normally...

http://sphs.spusd.net/groves/images/demos/cabbage1.jpg

A 2% phenolphthalein solution in ethanol is used as an indicator. At a
pH of 9 it is an intense pink color. At pH 7 it is colorless. You can
make a pH 9 solution with a little baking soda (sodium bicarbonate) in
water and neutralize it with acetic acid (vinegar) to decolorize.

I am a textile artist and dye all my own material- I could help you if
you specify what substance you want to use the color on- there are
many concentrated forms of color that are formulated for use on
different surfaces.

The color will be used on concrete and it needs to fade to clear
shortly after applying it.