The nature of the vestment colors is an interesting case all its own. Vestment colors, while theologically based, are essentially Tradition.
Byzantine Tradition has two "defined" liturgical "colors"... Light and Dark. Specific prescriptions for which light colors and which dark colors are used are essentially preferential norms.
http://orthodoxwiki.org/Liturgical_colors has an excellent article on the Slavic use, listing:
The most important feasts of the Orthodox Church and the sacred events for which specific colors of vestments have been established can be united into six basic groups:
1. The group of feasts and days commemorating Our Lord Jesus Christ, the prophets, the apostles and the holy hierarchs. Vestment color: gold (yellow) of all shades.
2. The group of feasts and days commemorating the most holy Mother of God, the bodiless powers, and virgins. Vestment color: light blue or white.
3. The group of feasts and days commemorating the Cross of our Lord. Vestment color: purple or dark red.
4. The group of feasts and days commemorating martyrs. Vestment color: Red. [On Great and Holy Thursday, dark red vestments are worn, even though the church is still covered with black and the holy (altar) table is covered with a white cloth.]
5. The group of feasts and days commemorating monastic saints, ascetics, and fools for Christ. Vestment color: green.
The Entrance of our Lord into Jerusalem (Palm Sunday), Holy Trinity Day (Pentecost), and Holy Spirit Day (Monday after Pentecost) are, as a rule, celebrated in green vestments of all shades.
6. During the Lenten periods, the vestment colors are: dark blue, purple, dark green, dark red, and black. This last color is used essentially for the days of Great Lent. During the first week of that Lent and on the weekdays of the following weeks, the vestment color is black. On Sundays and feast days of this period, the vestments are of a dark color with gold or coloured ornaments.
I've seen Pentecost in Gold, not green, but that may just be a local variation; I know that most of the priests in question have green vestments.
A byzantine cleric needs at least two sets of vestments, with white and dark red being typical. Next most add Gold and Light Blue. Some add a bright red, some purple, and some green next, and those who can afford to add all...
But realize, for a subdeacon or deacon to have the full panopoly (Gold, White, Purple, Dark Red, bright red, Black, Green, Light Blue) one is looking at at least $3500, and usually closer to $5000. And while the talented can make them, materials alone are still $100 per vestment set for good looking brocade and the requisite trim. And a priest will be looking closer to $10,000. for the same array.
Now, the Romans have a simpler, more Rigid system. White, Gold, Red, Purple, Rose, Green.
Green is for ordinary time, the default.
White for the Christological and Marian feasts, and funerals
Purple for penitential functions, feasts and seasons.
Red for Martyrs feasts.
Rose is optional for the "reprieves" during penitential seasons (Gaudate and Latare sundays).
Black is optional for certain uses normally calling for purple; it's usually trimmed in purple.
Gold is the "substitute" for any but purple.
Blue is permitted in some provinces as an option for marian feasts; in others, white with blue trim is used as a personal devotion of the clergy (because blue isn't authorized).
So a Roman must have, as a minimum, Purple and Gold. Some missions have but these two, for what can't be done in gold must be in purple. Most have White, Green, Purple, and Red. The truly expansive have white, green, purple, red, gold, black, and white with blue trim.
And the some of the other rites have it simpler still: White with Red Trim, all year round. Some add gold, as well.
It's all discipline of the local church, based in theology and traditional views of the meaning of colors.
Red for blood, white for purity, Gold for the treasures of the Faith. The rest vary too much by local tradition for blanket statements.