One of the not so common effects you can apply to your photos is the threshold effect. This effect works the following way:
In the photo threshold effect all the pixels with color values lower than a defined threshold (which you define) will be changed to another defined color (which you define as well) creating an interesting effect. Commonly all the pixels that satisfy this condition usually are gather in the same place or belong to the same region, even this region is not closed, this way what will happen is that some of the homogeneous regions in the image will be switched to the color of your selection.
For example the next photo was thresholded (filtered) with a red filter