Going to Lab color for any reason requires a high bit depth or you risk image degradation. 16 bits minimum are needed to keep this from happening. To create your luminosity masks all you need to do is create duplicate image layers with the channels required for the mask and select create a selection based on those combined channels. For example, to isolate luminosity in an RGB image, there's no need to convert to LAB, simply Command-click (Mac) the RGB channel to make it a selection. This selects the luminosity. So to take that method and answer your question, to create the luminosity of cyan, you need the Red channel added to the Blue channel which creates cyan then Command-Click for its luminosity. Set Non-linear History in the History palette, Then Using this same method you can separate out each color until you have what you need. and fill from history to get back to the orig image.
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