This is a tough question, because there's no universal standard for the color of gambling chips. Sort of like paper money in different countries, gambling chips of the various denominations are different in color from jurisdiction to jurisdiction and even casino to casino.
However, some generalizations can be made.
First of all, the New Jersey Casino Control Commission has standardized casino chips in Atlantic City, so that's a good place to start. In A.C. casinos, dollar chips are white, $5 chips (also known as "nickels") are red, $25 chips (also known as "quarters") are green, and $100 chips are black. Chips worth $500 are purple (aka, for some reason, "Barneys"). This is pretty much universal in Nevada as well.
From there, though, things start to vary. Atlantic City has $2.50 chips used at blackjack (and sometimes poker) tables; they're pink. A.C.'s $10 chips are blue and the $20 chips, used primarily at the baccarat and pai gow poker tables, are yellow. Thousand-dollar and $5,000 chips are orange and gray, respectively; both are oversized. Atlantic City's $10,000, $25,000, $50,000, and $100,000 denominations are plaques that must be marked with serial numbers.
In Nevada, dollar chips are usually white, like New Jersey, but they're sometimes blue as well. Chips worth $1,000 are often yellow (aka "bananas"), though they're sometimes orange (aka "pumpkins"). Denominations higher than that are all different colors, depending on the casino. Also, casinos frequently issue special limited-edition commemorative chips, which are highly collectible and feature multi-colored images relating to the event they signify.
Europe uses similar color schemes to the $1, $5, $25, $100, and $500 standards in the States, though there's some variation; for example, some casinos use pink for two-euro chips and blue for €10.
This should get you started in recognizing chips. When you see a chip of which you're not sure of the value, it'll almost always be $5,000 or higher. You can ask someone nearby if you've just got to know.
