I'm not an expert on this matter but is there a requirement of some sort regulating the color of the outfit. I rarely see any other color than black and I'm not talking about localized variants of the clothing. Given that black absorb more heat than many other colors, it would seem to be counter-intuitive for such a color to be so common in a hot environment from where niqab/abaya claim to be originated from. Does the color have any significance to it? If so, is it cultural or cited in Islamic sources?
The colour of the niqab/abaya or hijab is in first place a cultural and a custom issue, which is not related to Islam. A woman may wear clothes in any colour as long as these don't attract too much attention and fulfil the requirement of an adequate clothing as prescribed in the Qur'an and sunnah.
For example wearing a red 'Abaya in a location where the custom is rather black maybe considered as wearing clothes attracting attention. Some scholars therefore consider wearing colourful or pattern clothes as prohibited for women.
Nevertheless some scholars consider the following hadith from sunan abi Dawod as an evidence that the wives of the sahaba were used to use black coloured garments:
When the verse "That they should cast their outer garments over their persons" was revealed, the women of Ansar came out as if they had crows over their heads by wearing outer garments. (Sunan abi Dawod)