Roof valleys can be open or closed. The closed variety is where the tiles of two slopes meet whereas an open valley has a strip of metal flashing where the two slopes meet. If installed correctly, both types of valleys will work fine at keeping moisture on the outside of the roof and look beautiful at the same time. Much of that beauty comes from the types of roof tiles that are used. For example, if you use cheap roofing tiles that come in just one color blend, you probably aren't going to come up with the prettiest roof on the block.
Natural slate and wooden products can look fantastic, but they come with their own set of disadvantages and might not provide the colors you are looking for. In addition, to get those color blends, you are going to have to spend a huge amount of time just sorting through them. However, use polymer tiles made by DaVinci and you won't have to waste precious hours of your time in your search for the right color combination. Every open valley is going to look gorgeous because bundles of DaVinci tiles come in pre-sorted color blends.