If you compare the layer with a wall with windows in it, the Behind paint would be applied to the outside of the house, and only be visible through the windows. As a matter of fact, all RGB colors with max values for two channels are more or less bright versions of a certain process color, no matter what the third value is. Using pure Red, Green or Blue produces similar results. Screen , Addition and Lighten may appear very similar, sometimes almost as the same mode, but there are certain important differences. Lighten Only compares the FG and BG RGB-values, and chooses the higher value for each channel.
Addition just adds up all values, and thus comes up with a brighter image.
Compare it to putting two scales of equal length on top of each other. The composite value can be found somewhere on the FG scale, and that placement is determined by the other scale. The most obvious difference between the three modes is that Screen color is darker than Addition , but lighter than Lighten Only . This is because Lighten is seldom affected by dark colors, it'll just choose the color that is lighter in all RGB values if there is one. These two modes often result in similar outcome, where Multiply is always a little darker .