Ahl al-Bayt

This is referring to Qur'an 36:12 and there is some discussion here:

https://www.al-islam.org/ask/what-is-the-full-significance-and-historica...

If the phrase "imam mubeen" is understood to refer to a human Imam, such as Imam 'Ali, there is no reason why it has to refer to a human Imam who is presently living since the souls of humans exist after death, and the Qur'an states that martyrs are alive. Although, of course, it could, and God knows best. 

For the most part, we don't have strong historical evidence about the details of what women from the Ahl al-Bayt (A) were wearing, apart from a few situations where there was a reason to mention a specific fabric or something.

Anyway, without being an expert in the history of clothing, it seems that in some places, the tendency to wear black is pre-modern (and I suspect due to convenience) and in some places it is a product of modernity. When discussing today's "Islamic fashions", it is worth keeping in mind that what we have access to today is largely due to industrialization - for instance, the production of synthetic fabrics, cheap stretchy socks, and mass-produced garments - and people in the past did not have these things. Also, the concept of global "Islamic fashion" (often epitomized by the black Saudi-style abayah) is relatively new and probably due to globalization; up until recent decades, Muslims seemed to tend to wear their own regional styles more. 

(There are, however, some interesting books about the history of clothing in the Muslim world, if it is a subject of interest, as well as some early photographs)

To my knowledge, in places like Iran, in the past, it was more common to wear coloured chadors, but in the 20th century, black became the popular colour. 

Anyway, shari'ah in and of itself does not regulate what colour you have to wear. However, a general principle is that of 'urf; that is, one should dress in such a way which is considered normal in the area and does not attract attention (unless there is a clear reason to do otherwise, for instance, everyone may be wearing a bikini on the beach, but that doesn't mean it's appropriate to wear a bikini). So colours are one of the matters of 'urf; in some places, it might be normal and appropriate to wear bright colours, and in other places, such as Qom, it might attract a lot of attention or be an inappropriate form of social signalling. Non-Muslm countries also tend to vary widely in their 'urf regarding the colours people wear so it is really left to one's individual judgment, or you could ask people around you if you are concerned. 

Anyway, as is said, Allah is beautiful and loves beauty, and Allah created all the colours and did not make any of them inherently taboo.*

(*apart from cases where wearing certain colours is specifically considered makruh, this is not the main subject here)

The 12 Imams are not exactly like the Prophet Muhammad in that each of them were separate individuals and had his own life, experiences, personality, features, etc. (That is, they did not share the same soul.)

However, they all shared the same teachings of the Prophet (S). This is the job of the Imams (A) is to correctly explain the teachings of the Prophet (S). According to dominant Twelver Shi'i belief, knowledge of the correct Prophetic teachings is a divine gift transferred to one Imam after the death of the previous Imam; therefore, they do not make mistakes in the teachings or disagree about what the Prophet (S) taught. 

Also, the Prophet and 12 Imams are all considered sinless. They all obviously also shared a strong commitment to Islam.

Maybe this is one reason that they tend to appear the same - it is our flaws and faults that oftentimes distinguish us the most from each other!

The rational reason for this is that Allah appointed them to preserve and expand upon the message as was necessary. If you think about it, most Muslims came to Islam in the latter part of the Prophet's life. Many of them really didn't have enough time to learn much about the religion at all, especially because the Muslims were busy fending off attackers and dealing with a lot of practical things. And after the Prophet, not all the Companions agreed on everything. So it makes sense that if Allah is going to appoint a prophet with the final message that Allah would be sure that message is protected; this is done through the imamate.