Jeans are muy importante. In my opinion, you can never ever have enough. There's so many different ways and uses in which to wear them that the possibilities are almost endless.
Let's start with the essentials. Black denim is a necessity, to be blunt, it's one of the coolest looking jeans that you can own even if it seems simple. Dark denim is also essential because it can either be worn casually with a tee, or more formal with a button-down. Light is primarily for a more, as you may have guessed, a relaxed-light look.
Now, all the colors in between is where you can have fun. Colored denim is great for making what could be a subdued outfit stand out. Whenever you do one of these, make sure that the colored denim remains the center of attention. If you have too much going on, it will actually take away from the look no matter how cool it is.
It's also fun to get medium washes that are somewhere in between Dark and light. You can do washes and correlate them with shirts. For example, if you have a medium wash jean with a tan wash, wear a brown tee to bring that out. If there's a white wash, wear yellow or light blue since those two look great with white. Whenever a wash or lining is involved, that then takes precedence over the jean itself.
By collecting a lot of jeans, it opens your wardrobe to new heights. Great jeans to look out for are gray one's because they go well with just about everything, and jeans with some shine to them. Not in an over the top way, but in a subtle and significant way.