What is the difference between siyah and kara when used as an adjective to describe an object as having the colour black???

My understanding is that kara is used more in the sense of dark, gloomy as opposed to siyah used simply to describe a dress, bag, coat, whatever.  I would like to know if I´m correct in this! 

There is no difference between "siyah" and "kara". In Anatolia,"kara"  is more useful than "siyah"

BLACK = SÝYAH = KARA

 Thanks harpoon.

You are welcome.

my turkish teacher said that the only REAL difference is that siyah is not turkish word but taken from arabic or persian (hmm...i don´t remember which one) and that kara is more older word for black...kara+kum, for example...black sand...

 There are more differences than this one...Why you dont say Siyah deniz? And Kara also land...for ex. Kara Kuvvetleri....

Siyah is probably from Persian. black in Arabic is aswad

 We dont interrogate the languages in here. We dont say SÝYAHDENÝZ in Turkey, because name of the sea is KARADENÝZ. Yes, you are right for other meaning of KARA that meaning of LAND. Like other foreign languages, Turkish has "HOMOPHONE WORDS" also.

kara is the originally Turkish word and siyah is from Persian. Although they have the same meaning, there are preferences in some cases. This is usually the case in Turkish. There are words originating from different languages and meaning the same, but one is preffered in one case and the other is preffered in another.

I agree with LIR, mostly (I say mostly, you can actually use them interchangably) to describe the color of dress, bag, coat, whatever, we use "siyah". It is less common to say "kara elbise" than "siyah elbise". However, when we are talking about dark, sky, eye color, or a black dirt, we use more commonly "kara".

e.g. kara gözlü, kara kaþlý

hava karardý (here it is not the word "kara" but the verb originating from it:kararmak)

ellerin kara olmuþ! ("your hands became black!" e.g. after a child played outside)

etc, etc...

anyway, as harpoon also said in Anatolia it is more common to use "kara" in any case. So don´t hesitate to use them interchangably (except for proper nouns like KARADENÝZ(black sea) or KARADAÐ(montenegro) ). However if you hear the language some time you can easily "get" the preferential uses.