As a general rule, the darker the tea, the stronger the flavor and higher the caffeine. But "darker" doesn't always mean the color of tea will be dark when steeped. With oolong tea, for example, some of the finest varieties yield a very pale brew, though the leaves themselves are dark. Oolong tea can range from yellow or pale brown to bright green.
And with white tea, sometimes the steeped color is very dark, especially when more-developed leaves are used. In the traditional Bai Mu Dan variety, we use leaves that are 2 days old (the tips) as well as up to 4-week old leaves to achieve the full flavor of natural white tea. The resulting tea, when brewed, gets darker with the second and third steepings (or if you steep it for a long time).
White tea can be surprisingly dark when steeped; this is the natural color and flavonoids that are preserved deep within the leaf. If just the needle tips are used, it tends to be light in color, but with much less flavor than the full-leaf varieties.
Green tea can be very pale or dark, depending on the variety. With all teas, how long you steep the tea and the quantity you use will determine a lot of the color, and hotter water tends to make the tea darker more quickly.