Why do all the colors change for their nouns except orange?
As in un vestido naranja. Why doesn't it change?
4 Answers
I think that's because the proper name for orange is really anaranjado, and using the fruit name is just a convenience. The fruit is a noun with its own gender, and doesn't change, even when it's conscripted for use as an adjective.
Maybe a native speaker can confirm this...
I think Annie was joking.
But anyway, see this article at about.com.
Most notably this part:
Generally, the invariable adjectives are that way because they are thought of as nouns (as are la hembra and el macho), and they include the colors that come from names of things; esmeralda (emerald), mostaza (mustard), naranja (orange), paja (straw), rosa (rose) and turquesa (turquoise) are among them. In fact, as in English, almost anything can become a color if it makes sense to do so. So café (coffee) and chocolate can be colors, as can oro (gold) and cereza (cherry). In some areas, even the expression color de hormiga (ant-colored) can be used as a way to say something is ugly.
Consider the following rule:
While English nouns can function like adjectives when placed before other nouns, Spanish nouns must be preceded by a preposition in order to modify another noun.
Un libro de español - a spanish book *
Una mesa de vidrio - a glass table
Una bolsa de papel - a paper bag
Una casa de adobe - an adobe house
*Un libro de español is a book for learning Spanish. Un libro español is a book written, manufactured, or published in Spain.
And then note Lazarus's additional comments:
However, there are a few cases where you can put two nouns together without a preposition:
when one noun is the name of the other, as in "río Amazonas".
with colours: color café, color piedra,...
certain fixed terms like hombre rana, cartón piedra,... (not many)
Notice that he says that colors are the exception to this rule. Of course we all assume colors are adjectives not nouns, since in English I think that this actually is the case, but in Spanish colors like Orange, café etc. are considered nouns. (though some of them are considered adjectives)
It's because it's an unbecoming colour, not much used by fashion designers, not like leopard skin, or Royal Purple. It makes you look pale and sickly, especially when combined with green.
On the trees it looks fine, for some reason. 
Pesta is right. Colors named after objects, like fruits or minerals, doesn´t change their gender, and sometimes doesn´t change their number, like café, violeta, malva, naranja:
vestido naranja (no naranjo), blusas violeta (no violetas), etc.
