You can add several kinds of effects to the same clip. For example, you might have a "Fade In From Black" effect, a "Watercolor" effect and a "Mirror" effect all added to the same piece of video.
However, some effects cancel each other out. You can't have a "Grayscale" effect and a "Sepia" effect on the same clip, for example, because Grayscale makes a clip black and white and Sepia tints it brown, and you can't have a clip that's black and white at the same time as being brown. When there are two conflicting effects like these, the effect you put on last will be the one that is displayed.
