Figuring out Blend Modes .. ArgH!

Question

I've read numerous tutorials and books on the subject of blending modes and the one thing that bothers me is understanding how they work with color to the point that I understand and can remember forever, without having to always toggle though all the blending modes to find the right one when I'm working.  I'm hoping someone / some where can explain it in a way that can be understood where as, when I'm working I can just select the specific blending mode I want without toggling though most of them ?

Answer 1

StrongBeaver,
Be easy on yourself. Start by understanding one single (but representative) blend mode. That will give you the concept for other blend modes. The MULTIPLY blend is a great starter. Understand it and you're on your way. Below is a cat-and-dog-language explanation of it.
First, just what is being multiplied? It is basically color intensity (called color value).When rabbits multiply they INCREASE in number. Yet, when color values multiply, they DECREASE in value. Counterintuitive or a misnomer? Not if you realize that FRACTIONS are being multiplied.
Color value is popularly described on a 0-255 scale, with 255 being the maximum value---limited by the eye's capacity. In blending algorithms, the intensity scale is "normalized", as they say, to 0-1. Thus, 255 becomes 1, 192 becomes 3/4, 128 becomes 1/2, etc.
If you have a red pixel of value 128 and multiply it by itself, you get 128/255 x 128/255 = 1/2 x 1/2 = 1/4. Since we like to verbalize color values on the 0-255 scale, 1/4 translates back to a value of 64.
I mentioned a "red" pixel above. But each pixel has THREE associated color values (red, green,blue). The multiplication takes place separately and independently in each of these color "channels". When two layers are multiplied, the red value at a given pixel site in one layer is multiplied by the red value at the same site in the other layer. Ditto for blue and green.
Saying that "two layers" multiply is not wrong, but it is too restrictive. Only the upper layer is put in the multiply mode. The pixels in that layer multiply with WHATEVER NET color value is visible BENEATH it. That could be another single layer or it could be a conglomerate of layers blending or otherwise interacting with each other.
More generally, then, one should say the upper layer blends with the substrate. It doesn't blend with anything above it unless that is called for by the above layers, Further, one can assign a blend mode to a paint brush, If Multiply is assigned, the color value painted on the layer multiplies with the existing color, again independently at each site and in each channel,
It should now be clear why Multiply darkens, The color value in each channel resulting from multiplication is always less than either inputs, except if one of the inputs is maximum, in which case the resulting color value is the same as the other channel value because that other value has been multiplied by 1.
The physical parallel to multiplying is a pair of filters, The first filter reduces white light incident upon it, passing a fraction on through which, in turn, is incident upon a second filter which passes along a fraction of the fraction.
You should next look at the "Screen" blend mode because it is so similar to the Multipl;y mode but produces the opposite result (lightens). In screening, the INVERSE*  color channel values are what get multiplied, and the inverse of that multiplication gives the resulting color value. The resulting channel value is always greater than that channel's input values (unless one of the inputs is 0 for which the resulting color value is that of the other input).
*In case the meaning of "inverse" eludes you, the inverse of a fraction (f) is 1-f. For example, the inverse of 1/4 is 3/4, and the inverse of 1/2 is 1/2. These inverses multiply out to 3/8. The resulting color value of 5/8 (the inverse of 3/8) is greater than either input (1/4 and 1/2), i.e. it's lighter.
Hey! that's two blend modes down. Or really three because the Normal mode is clear. Only 22 modes to go!