Different Grasses Grow In Different Seasons
There are two ways of categorizing grasses that I know of. Or at least, two ways that are important to me:
Growing season: Some grasses grow in cold seasons, others grow in warm seasons, which is why one clump of grass will be green in early May and another might still be brown.
Structure: Some grasses form clumps and send out runners to spread and create a new clump. Some grasses form sod, a dense mat of grass, roots, and last season's organic matter.
If you want an ornamental grass you can enjoy all year, pick a cold-weather grass that forms clumps. If you're closer to one of the Earth's poles, this will likely mean you'll get a native grass too.