I love a good gallery wall. And I know people can get stressed out trying to get frames exactly perfect in a grid, but the best way to get a great/cohesive look without all the stress is to just go for it. Hold the frames up to the wall, eyeball a mark and you're not going to be too far off. If you are, it's just a nail and you can redo it, fix the wall later. Design should be fun, should represent you, and not give you any headaches.
On a neutral wall, a great way to add color is to mix up your frames (style, size, color). The common theme will tie everything together. If you have a busy wall (wallpaper, a strong color), use the same color frames/mattes, but vary the size for a great look.