Online petitions hardly ever work.
And you'd need A LOT more than 500 signatures to change Ninty's mind.
It's all supply and demand. If there is little demand for something, then there will be little reason to supply it.
If they released all the colors you saw at E3, you'd prolly see a bunch of one or two colors missing while the rest sit on the shelf. Retailers (and companies) like their products to sell, so Nintendo prolly chose the two colors that they thought people would want to buy.
I'll admit, the other colors were cool, but we're prolly not going to get all of them. Remember all the different colors of Wiis whenever it was first announced?