First, I don’t too often see blue primarily used in side/back lighting positions. So I assume you’re probably referring to straight plays or extremely realistic musicals. Otherwise, using one standard color all the time is a poor way of using color to the designer’s advantage. First of all, when working with realistic type things, you work with not only color but heat. When looking at the color spectrum blue light is around 5400 K where-as
daylight is about 5700 K which is actually a shade of blue though we think of it as being a shade of yellow. Therefore, using blue light as back and side light keeps more of a realistic look, appearing more like
daylight. Though there could be a million other reasons as to why people choose blue. A not so saturated blue also looks a lot like white light and people won't notice the color as much, though I don't know why people would use a low saturated blue for a side and backlight color.
We use a different color for backlight than the colors for front light because backlight is used not only to highlight and give the actor a 3rd dimension of light but also add a kind of color-wash to the
stage floor and a contrast which separates the performer from the background. Without contrast, a
stage picture appears very dull and lifeless. Usually the backlight tends to be more saturated than any of the other of the positions (thought it depends on the designer) because the color doesn’t alter the look of the performer or costumes in any harmful way while helping add mood to the scene.
Also, if you’re using side light to enhance the warm and cold colors of your show you usually pick a more saturated version of the front light colors you used. However, when designing for moods your sidelight and backlight colors tend to portray the over-all mood or “look” you’re going for and not any particular color. The same idea of contrast applies to the sidelight like it does to the backlight, though side light colors tend to be more saturated then front light but less then backlight. So light backlight sidelight is used to add contrast and color to the
stage but while also lighting the actors. However, people use sidelight for different things. It all depends on preference. Usually high side light is more of a natural color then side light used to
throw color.
A designer chooses colored light for one of four reasons:
-Light is motivated by a specific source (i.e. the sun)
-To reinforce the mood of the scene
-Visual contrast between light sources is desirable, color helps this contrast
-Change or dramatic
effect is desired
However, like I said, I don’t see people using blue all that often (yes they use blue, but not in the context that I believe you’re referring to), but I work with a lot of theatres that do musicals rather than straight plays. In the case of any theatrical production, you usually want color; a designer that can’t design with color won’t make it very far in the lighting world. Now don’t get me wrong, I’m not saying use odd colors to do a show, but rather colors must be considered, using a standard color scheme for every show won’t get you very far.
There really is no set way on what colors to use (which is a good thing) people who use the basic color scheme are usually not comfortable with color mixing, or just believe that is the correct way to do it. It all depends on the designer and their whims (and of course the director has some say). However when dealing with realistic plays I can see how one would use blue light, it is the safest way to go and the most natural looking (assuming you’re going with a less saturated blue). Though I personally wouldn't and don't rely on one color as my go to color for everything. Before choosing color a designer must consider the over-all emotion and style of the show. Lots of blue light in a comedy may be considered somewhat out of place, where-as in a more dramatic show it may fit better because blue is considered a cool (cold) color, though a lighter blue (less saturated) tends to denote more of a tranquil feeling.
So basically, people use blue because it’s safe and use different colors for the side/backlight to add contrast which livens up the over-all look of the scene.
Hopefully what I wrote made sense. I may have left out some important stuff which has slipped my mind. But honestly, there is no right or wrong answer to you’re question, it all depends on the designer, and what they feel looks and works best with what they’re trying to portray. Looking at color in a scientific way and only a scientific way (of course it's science but don't just look at it like a science) is only going to hurt a designer. Sometimes thinking, well this is the same temperature of natural
daylight or night light is not the correct answer to a color choice unless of course you're working with TV or Film, in which case temperature is extremely important to consider. The most important scientific approach to color choice in live
theatre is how the colors affect one another and how they work together. And yes I realize color is temperature. By choosing temperature you're choosing color, but normally in live
theatre you look at gels as colors not as temperatures, as how the audience will perceive the color, as what emotional quality it will portray.