LIE Editor options question

Hi, 

  I have a question, can someone point me to a short description/explanation of the various "blend modes" in the LIE editor ?

  Specific interest, making something as a layer above another layer, withouth complety covering the layer below (the "source over" option), and without having to modify the layers so they have an alpha channel - I have already done that, and it works, but the idea is to really show something has having been put on top of something else, so covering but kind of adding up on top of what is already there in a way that covered yet not covered (tried the "add" in the LIE editor, but did not seem to do what I expected).

  It's about putting some kind of skin/painting, on top of the character skin, and on top of a tattoo. For the tattoo, I have made it as layer with alpha and used the "source over" on the skin, and works fine (anyway the tattoo is meant to cove the skin below).

   For that painting, I modified a texture that had the painting so I got the painting with alpha without the skin, and can put it on top of the skin I want and the rest, but I think it would look better if there was some way in which instead of the painting alone with alpha (which come out kind of too perfect, almost like if it was wall painting), I could use the skin that has the painting on top of the original skin, but without hiding what is below. 

   I saw there is an "opacity" value for the layer, but I am afraid that would make kind of too uniform.

  Thanks