First of all, there are a bunch of blending modes; but most of them are almost never used for texture work; (except for really specific cases or special effects). 99% of the time you'll use only a few; so you can skip the others you won't miss a lot.
(However I'm not sure which ones are supported by Plasma)
Those that are used the most are:
Mix : This the default one. The texture is treated 'uniformly'. If you change the opacity the whole textures become more/less transparent without any special effects.
Screen : only the light part of your texture show up. If you use a black and white texture, only the white part will show up; the darker parts will be increasingly transparent. Useful when you want to use the lighter parts of a contrasted texture and you dont' want to spend time with an alpha mask. (it is often used for light effects; all things with energy, fire, light flares, etc)
Multiply : the inverse of Screen: only the darker parts of the texture show up. Very useful when you want to add a dirt/shadow texture easily.
Overlay : it is a combination of Screen and Multiply. So the 'middle' (gray) is transparent. Useful in a variety of cases when you want to blend a texture easily. (It is also used in Uru for lightmaps where you need to mix both the light and dark part of the texture)
Add : is similar to Screen, but it makes your texture much brighter. Very useful for light effects. For instance for light flares sprites: if you look at the Cyan textures you'll see their light flares sprites textures always have a black background; no need for an alpha mask. (this mode is called Linear Dodge in Photoshop.)
Value : basically it only uses the Luminosity information of the texture, and it shows the colors of the textures under it. It is sometimes used as an alternative to Overlay; when you don't need the colors of the texture. (it's called 'Luminosity' in Photoshop)
Color : it's the opposite of Value/Luminosity; only the color information of the texture is used. You use it to change the color of a texture bellow. Say you have a picture of a red car, you just put a blue texture on top of it using Color mode to turn it blue.
The other ones:
Lighten & Darken: only the parts of the texture that are lighter or darker than the texture bellow show up. This is actualy slightly different from Screen and Multiply because it creates a more abrupt transition between the opaque and transparents part of the textures. It's hard to explain. :/
Substract: like Multiply, but much darker. (Linear Burn in PS)
Hue: It's very similar to the 'color' option; and to be honnest I'm not sure what's the difference. :[
Saturation: this one is a bit unusual. You almost never use it but what it does it make
Difference and Divide are almost never used for regular work. Other exotic modes in Photoshop, like Pin Light or Exclusion, can be ignored too..
The best way to learn how those work is really to try them all; some are difficult to explain. Use two very different textures in two layers (if you use the same texture twice you won't see the difference), and test all the blend modes on the texture in the top layer. Change the opacity too, to better see the effects.
If you want to learn more about all that, there is a long (6 pages) article
here. It is quite detailed but just the 1st page should give you a good overview of the various blending modes.
by bnewton81 » Thu Sep 08, 2011 12:33 pm 
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