A Pink Pig ; Un cerdo "rosa" ; why does the colour end in " a "
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Question: I came across a simple phrase that I thought at first was simple! It was un cerdo rosa. A pink pig. Why does the color end in an a? I thought it was a feminine color? I notice "pig" has a masculine article and the pig is a male.
Answer: Rosa only looks feminine. In this case it's really a masculine adjective.
It's true that the vast majority of adjectives ending in -a are a feminine form of an adjective ending in -o or, less often, some other ending. But there are a few adjectives that end in -a in their "natural" state (i.e., in their singular, masculine form), and they don't change with the gender of the noun they describe.
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As you might have guessed, rosa is one of those. We say la flor rosa for "the pink flower," but we also say el carro rosa for "the pink car."
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Of the adjectives that naturally end in -a, and there are very few, most of them are (like rosa) derived from nouns. They include homicida (homicidal), hipócrita (hypocritical), and suicida (suicidal). Some adjectives of nationality, such as croata (Croat), also end in -a and are used with both males and females.
Probably the most important adjective that naturally ends in -a is cada, meaning "each." E.g., cada hombre debe salir, each man ought to leave.
If you're wondering whether an adjective ending in -a is feminine, look it up in a dictionary. Dictionary listings for nouns and adjectives are in the singular masculine form.
4 Answers
I am sorry Ray, that is not it, in the least.
Yes, rosa is femenine, however the subject in your phrase/sentence is not rosa, and that's the diff.
And FWIW, in Chile we would've said "Un chancho rosado", although "rosa" is understood but never used as a color.
c?
EDIT: I am not sure why I answered the way I did, I cannot remember what triggered that answer. But, I see I made a mistake concerning Rosa being femenine.
Some colors can be used as a masculine gender or as a femenine gender.
Verde, you cannot associate with any gender. In the cas of rosa, you can't either.
Rojo or roja Blanco or blanca
I don't deserve the best answer, though... 
Muchas gracias Ray. Una buena explicación.
I seem to remember being taught that colour adjectives which are already defined as nouns are not to be declined either in gender or number. These include: lila , naranja, púrpura, rosa and violeta.
I would appreciate a more experienced opinion on this please.
Me parezco recordar que enseñaba que los adjetivos colorantes ,que ya habían definido como substantivos, no deben de declinar, ni en género ni en número.
Estos incluyen : lila, naranja, púrpura, rosa y violeta.
Apreciaría una opinión mas experta sobre de este por favor.
Chileno , I am not sure what it is you are telling me, are you saying that pigs are not pink , or that is not how to say it in Spanish ? I think that the example was only given to show
that there are some adjectives which although feminine do not change their endings , but I appreciate your input , I am trying to learn here from experts like you , so thanks.
You are probably referring to white pigs. White skinned pigs appear pink due to the blood vessels other underneath. Their skin is similar to humans. And pig skin is sensitive to light and chemicals. That is one of the reasons they soak in mud. It helps protect their skin from the suns rays, it helps keep them from getting too hot, and it helps keep insects from biting them. Pigs are also often used to test skin products (make-up, lotions, etc) since their skin is so sensitive and similar to humans.
