Making GUITexture background alpha

Hi, I'm doing some textureoverlay technique which involves displaying GUITexture and rendering them to a rendertexture, using Camera render. But Everytime I render a smaller texture over a larger One. The larger One seems to be affected by some background color of the GUItexture element of the smaller one.

He color dont seem to be able to be turned off, resulting in an added color to the overall texture I want to create (except where the smaller texture was overlaid of Course)

Any hints? Thanks in advance