The darker "half" triangle indicates that the end of a clip is unfaded. Two clips butted together with no fading results in a dark full triangle. A lighter half triangle is a clip with a fade that is too small to be seen at the current zoom level. If you zoom in far enough you should be able to see the fade. The light triangle lets you know the fade is there when you're zoomed out. It looks like if you have a faded clip butted up next to a non-faded clip that you might only be seeing the darker triangle which will look like a half triangle compared to the others where both are the same color.