Blue is a color that can be dark or light but is usually not as saturated as red. In nature, warm hues tend to advance and cool colors tend to recede. Unlike yellow, blue is at its most intense when it is darker.
RGB Blue on the other hand is both warm and more saturated than RGB Red, and very saturated to the point that it's not usually presentable on its own in most cases. It is more than dark enough to read against white, but what if you want a blue that is easy to read against black?
Another thing to note: If you take the pure RGB blue and lighten it with even amounts of red and green you'll get a more purplish blue. And that would start becoming more purple at lighter hues. That would explain why LAB color space considers that blue hue to be more purple.
This is where the green channel comes in. Adding some green makes the blue look slightly duller (And LCH pickers will tell you the same thing) and a lot cooler. A lot of light blues have more green and are brighter when they do. It may have something to do with being closer to web cyan, which is a very bright color as well.
General Rules
- Because the RGB blue leans purplish, the green channel should be greater than zero at all times. It should always be greater than the red channel as well.
- The red channel is optional for darker blues.
- Generally, keep the green channel greater than the red channel if both are greater than zero.
- Because the RGB Blue is the darkest primary and very saturated, many darker colors with the blue at 100%/255 will look oversaturated as a result. This means that generally you should never use colors with only the blue channel active.
Examples coming soon!
Indigo
Did you know that Indigo was only introduced to the rainbow by Isaac Newton? While orange stayed on the rainbow indigo was later removed from some teachings because it was not distinct enough from blue. But this is a redder/darker subset of blue.
Examples coming soon!
Cyan
Okay, this is more like the printer color, not the web color. A lot of people in the art circles say that this is the kind of primary recommended for mixing paint, as proven by printers, which is ironic since the pigment in question is called phthalo blue and needs a lot of dilution or white to be considered cyan. But yes, the print cyan is more bluish than the web cyan, so I'm calling this a blue. Not THE blue.