‘Abdu’l-Bahá wrote:

“…The garden which is pleasing to the eye and which makes the heart glad, is the garden in which are growing side by side flowers of every hue, form, and perfume, and the joyous contrast of colour is what makes for charm and beauty. So is it with trees. An orchard full of fruit trees is a delight; so is a plantation planted with many species of shrubs. It is just the diversity and variety that constitutes its charm; each flower, each tree, each fruit, beside being beautiful in itself, brings out by contrast the qualities of the others, and shows to advantage the special loveliness of each and all.

Thus should it be among the children of men! The diversity in the human family should be the cause of love and harmony, as it is in music where many different notes blend together in the making of a perfect chord. If you meet those of different race and colour from yourself, do not mistrust them and withdraw yourself into your shell of conventionality, but rather be glad and show them kindness. Think of them as different coloured roses growing in the beautiful garden of humanity, and rejoice to be among them.”
(Paris Talks by ‘Abdu’l-Bahá, p. 52-53)

“Consider the flowers of a garden: though differing in kind, colour, form, and shape, yet, inasmuch as they are refreshed by the waters of one spring, revived by the breath of one wind, invigorated by the rays of one sun, this diversity increaseth their charm, and addeth unto their beauty. Thus when that unifying force, the penetrating influence of the Word of God, taketh effect, the difference of customs, manners, habits, ideas, opinions, and dispositions embellisheth the world of humanity. This diversity, this difference is like the naturally created dissimilarity and variety of the limbs and organs of the human body, for each one contributeth to the beauty, efficiency and perfection of the whole…

How unpleasing to the eye if all the flowers and plants, the leaves and blossoms, the fruits, the branches, and the trees of that garden were all of the same shape and colour!  Diversity of hues, form, and shape, enricheth and adorneth the garden, and heighteneth the effect thereof. In like manner, when divers shades of thought, temperament and character, are brought together under the power and influence of one central agency, the beauty and glory of human perfection will be revealed and made manifest. Naught but the celestial potency of the Word of God, which ruleth and transcendeth the realities of all things, is capable of harmonizing the divergent thoughts, sentiments, ideas, and convictions of the children of men…”
(Selections from the Writings of ‘Abdu’l-Bahá, p. 291-292)