I saw on an Islamic site that it is encouraged for people to marry someone of a different culture; the reasoning is that this helps to reduce racial prejudice. Is this true? Please provide reference in the Quar'an.

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Suitability between spouses has to be taken into account. It has a considerable effect on marriage. Muslim Scholars have agreed that the suitability of religion is a must.      A Muslim woman can not be married to a non-Muslim man and a Muslim man can not be married to a non-Muslim woman except the women of the Book, provided they were chaste i.e. (not adulterers or fornicators).
A man known by his dissoluteness would not be married with a religious woman, since he might be unjust to her or he might force her to commit forbidden acts.
As for suitability in lineage the Muslim Scholars have two different opinions:
1. It could be considered a criterion in marriage. This is the opinion of a large group of Scholars. They based their opinion on a Hadith that the Prophet said: "Arabs are counterpart to one another and slaves are match to one another". Reported by Hakim. Imam Ibn Abdul Barr ruled that this Hadith is Munkar/Denounced (i.e. the narration reported by a weak narrator, which goes against another authentic Hadith,) or it is a fabrication. All chains of this Hadith are not correct so it does not stand for a legal rule.
2. Imam Malik and whoever agrees with him said that only the suitability of the religion is considered and nothing else. Many sound evidence support the latter opinion. Allah Says: {Interpretation of meaning): (Verily, the Most honourable of you with Allah is that who has At-Taqwa (God's fear)}. [49:13].
The Prophet said: "Allah's Messenger (peace be upon him) said, "When someone with whose religion and character you are satisfied asks your daughter in marriage, accede to his request. If you do not do so, there will be temptation on Earth and extensive corruption". Tirmidhi, Nasa'i and Ibn Majah transmitted it.
It is narrated on the authority of Abu Hurairah that the Prophet said: "A man marries a woman for four reasons: for her wealth, for her lineage, for her beauty or for her piety. Select the pious; may you be blessed". (Bukhari and Muslim).
Imam Ahmad and Abu Dawood narrated that The Prophet (peace be upon him) said: "Allah, Most High, has removed from you the pride of the pre-Islamic period and its boasting in ancestors. One is only a pious believer or a miserable sinner. You are sons of Adam, and Adam came from dust. Let the people cease to boast about their ancestors. They are merely fuel in Jahannam; or they will certainly be of less account with Allah than the beetle which rolls dung with its nose".
Imam San'ani said: "It is proved that Bilal (freed slave) married Halah Bint 'Auwf the sister of Abdur Rahman Bin 'Auwf" (From the tribe of Quraish).
Therefore, the basis is the religion; any other criteria should come after it. All other standards have both negative and positive sides such as beauty, lineage, wealth and difference in culture and race. But the criterion of religion is free from negative sides provided the intention is correct and the religion is pure.
Please refer to fatwa: 82114 for more details.
Allah knows best.