To mask a range of colors, adjust the Hue Range, Sat Range, Val Range, and Threshold settings.
To invert the mask, set Positive KeyON. Only the color (or range of colors) specified in the HSV Color Key parameters are visible, instead of masked.
When you are finished masking the label, click the arrow in the bottom right corner of the shader swatch to hide the Stencil texture and the Image file (and the Mask file if there is one).
To preview the label map and mask do a full render.
To position the texture on a surface, see Use 3D mapping to position a texture on a surface or Use 2D mapping to position a texture on a surface.