Islamic groups adopting a banner or symbol, and
    the ruling on using the crescent as a symbol
Question

                We want to form a Muslim group
      in a place where there are a lot of student groups in the university, including groups
      from other religions. We want to distinguish ourselves with something and to have a
      symbol, because symbols are important in distinguishing groups from one another, Can you
      advise us of a specific color or symbol that we can use for this Muslim students’
      group?
Praise be to Allah.We put this question to Shaykh Muhammad Saalih
    ibn ‘Uthaymeen, who replied as follows:
“If the Muslim students agree on something, this is OK, but
    if each Muslim group is adopting its own symbol, it is not right. The symbol or color that
    we recommend is bright green.”
Is there any daleel (evidence) for this color?
“No, but people like this color and in the Qur’aan it
    says (interpretation of the meanings): ‘Their garments will be of fine green
    silk’ [al-Insaan 76:21] and ‘Reclining on green cushions’
    [al-Rahmaan 55:76].”
Is there any indication from the reports that the Prophet SAWS
    (peace and blessings of Allaah be upon him) had white and black banners that these colors
    are mustahabb in such cases?
“No, we cannot use these colors, because these were the
    banners in that time, and each era has things that are appropriate for it. But black now
    is one of the symbols of the Raafidah, as is well known.”
The crescent is commonly used as a symbol by Muslims to
    distinguish themselves from Christian groups who use the cross as a symbol. Is it
    permissible for us to use the crescent as a symbol in our publications and on our banners,
    to distinguish ourselves from other, non-Muslim groups?
“I do not think this is right, because the crescent was
    originally a symbol that was placed on graves that were worshipped instead of Allah. We
    read this in an answer given by some scholars and leaders of Daw’ah. But we could use
    a picture of the Ka’bah as a symbol, because it is the Qiblah of all Muslims.”
In some countries they put a crescent on Muslim places of
    worship to distinguish them from Christian places of worship, which have crosses on top of
    them. They do this so that people can tell them apart from a distance, because the
    architecture is often similar. Can we say that putting a crescent on the mosques or
    minarets of the Muslims is bid’ah?
“It is as you say (i.e., we can say that). It is sufficient
    for a mosque not to have a Christian symbol on it. Symbols may be something that is
    present, or something that is not present. If the Christians have a symbol, namely the
    cross, then the fact that the Muslims have no symbol is in itself a symbol. (i.e., we do
    not need to put anything). 
And Allâh knows best.