This color is the presence of all the light in the visible spectrum. When it enters our eyes, it stimulates all of our cone cells that God made light sensitive. Snow and clouds appear white because almost all of the light from the sun is reflected by water (either frozen or liquid), with only a small amount of the visible spectrum absorbed.
White, in the Bible and in today's society, is typically associated with purity, things that are good, innocence, honesty, and cleanliness.
Appearances of the color white
In the KJV Bible, the word 'white' occurs seventy-five times, twenty-nine of which are in the New Testament. It is the most frequently mentioned pigment in the New Testament and in the word of God as a whole. It is the third most referenced color in the Old Testament, behind blue (50) and red (47).
The most frequent Old Testament Hebrew words referencing white are laben (Strong's Concordance #H3836) and tsachor (#H6715), which refers to its name, chur (#H2353), which refers to linen and laban (#H3835), which is to make something this coloration. In the New Testament, the Greek word leukaino (#G3021) is defined as whitening something, leukos (#G3022) refers to its name, and lampros (#G2986) refers to a radiant, magnificent version of this pigment.
The use of white lends itself to representing righteousness (Daniel 7:9, Matthew 17:2, Mark 9:3, Luke 9:29, Revelation 1:12 - 14, 6:11, 19:8, 20:11) or pretending to be so (Revelation 6:2), or made pure (Daniel 11:35, 12:10, Psalm 51:7, Isaiah 1:18, Revelation 3:18, 7:9, 13 - 14). It also symbolizes wisdom (Revelation 1:14), holiness or dedication to God (2Chronicles 5:12, Mark 16:5, John 20:12, Acts 1:10, Revelation 4:4, 19:14) or joy (Ecclesiastes 9:8).
The color of Prophecy
Revelation discusses two prophetic horses, which are both white and seem to represent truth and righteousness. This is the conclusion drawn from several Bible Commentaries (Adam Clarke's Commentary, JFB Bible Commentary, etc.). The truth is, only one of the horses is of God!
And I looked, and behold, there was a white horse; and the one who was sitting on it had a bow, and a crown was given to him; and he went out conquering, and to conquer. (Revelation 6:2, HBFV)
And I saw heaven open; and behold, a white horse; and He Who sat on it is called Faithful and True and in righteousness He does judge and make war . . . And He was clothed with a garment dipped in blood; and His name is The Word of God. (Revelation 19:11, 13)
Clearly, the horse of Revelation 19 represents righteousness and is rode by a glorified Jesus. The rider is clothed in a garment 'dipped in blood,' which is Christ's own blood shed for our sins, and his name is the 'word of God,' which is precisely what the apostle John calls Jesus (John 1:1).
What is the meaning of the first horse? First, Jesus does not need to receive a crown or be given the power (sometime in the future) to conquer or take control. Christ has already received the 'crown' of dominion over the entire universe (Matthew 28:18, Mark 14:62, Colossians 2:10, etc.).
Second, Jesus does not need to conquer anything, as conquering implies a struggle where either side has a chance (however small) of gaining victory. Satan has always lost against God and has a zero chance of ever winning even the tiniest of battles! He utterly and totally failed to get Jesus to sin just one time! In fact, the devil did not have the power to unilaterally lay a single finger against just one person (Job 1 - 2), but instead had to ask God for permission!
At his Second Coming, Jesus will assert his right to rule over mankind with a display of power the goal of which is to get people to repent (see Revelation 9:20 - 21, 16:9).
Additional info on Biblical Meaning of white
The horse of Revelation 6 represents the ability of Satan to deceive and seem like he represents God. According to the apostle Paul, one of Satan's primary tactics is to lead people astray by appearing to be "an angel of light" who represents the truth (2Corinthians 11:13 - 14). He is, however, the father of deception and lies, and a master counterfeiter. In the end time, God permits him (Revelation 11:2, etc.) to deceive all mankind into thinking he is their 'savior.'
The devil's goals, in the End Time, will be to kill all true Christians, have humanity worship and obey him, and gather an army to fight the return of Jesus Christ (see Revelation 13:4 - 7, 14:11, 16:12 - 16, 17:14 - 18, 19:17 - 21, 12:9)! Paul also tells us that if the devil can appear to be righteous or 'white' so can the angels who follow him (2Corinthians 11:13). The horse of Revelation 6 and 19 each represent something unique in Scripture.