Etherdelta: How the order book works:

[CryptoGirl]

Red orders are sell orders.
Green orders are buy orders.

If you want to buy (a token, and pay ETH for it), you need to click someone else's (red) sell order.
If you want to sell (a token, and get ETH for it), you need to click someone else's (green) buy order.

If you take a step back and think about it, this should be obvious.
The green orders have lower prices, and the red orders have higher prices. If you could buy the lower (green) prices and sell the higher (red) prices, you would do that all day and become fabulously wealthy. But that's not how the world works. So the green prices must be the ones you can sell, and the red prices must be the ones you can buy.

You also have the option of placing your own order in the order book.
If you don't like the best available prices, you can make your own order. If you want to buy, fill out the 'Buy' form and make your own (green) buy order. If you want to sell, fill out the 'Sell' form and make your own (red) sell order.