Anjous can be green or red (which are often labeled "Red Anjou"), but the color doesn't indicate any major flavor or texture difference, and they can be used interchangeably in recipes.
Look for Anjous that feel heavy for their size, with bright, taut skins and relatively firm texture when lightly squeezed with the palm of your hand. Never pinch or poke pears with your fingers to test for ripeness, since it will in all likelihood bruise the fruit.
Anjous are great for eating out of hand or for cooking. Anjou pears are usually available October through May.