BibTeX:
@incollection{AbdulBah1912,
  author = {'Abdu'l-Bahá},
  title = {All Colours Are One: The Master Proclaims Oneness at Howard University},
  booktitle = {The Promulgation of Universal Peace},
  year = {1912},
  publisher = {Bahá'í Publishing Trust},
  url = {https://www.bahai.org/library/authoritative-texts/abdul-baha/promulgation-universal-peace/},
}

APA:
'Abdu'l-Bahá. (1912). All Colours Are One: The Master Proclaims Oneness at Howard University. In *The Promulgation of Universal Peace*. Bahá'í Publishing Trust.

MLA:
'Abdu'l-Bahá. "All Colours Are One: The Master Proclaims Oneness at Howard University." *The Promulgation of Universal Peace*, Bahá'í Publishing Trust, 1912.

Chicago:
'Abdu'l-Bahá. "All Colours Are One: The Master Proclaims Oneness at Howard University." In *The Promulgation of Universal Peace*, Bahá'í Publishing Trust, 1912.