Elliott,
I am assuming what you want to do is combine your Luminance image with your RGB image. You will want to have built your RGB image in the first instance using the ColorCombination and then adjusting it with Background Neutralization and ColorCorrection in the linear stage. When you get to LRGBCombination then, you simply load up the Luminance image and then drag the little triangle in the lower left of the toolbox unto the RGB image and it creates the LRGB image from those.
Hope that helps.
Jim