Luminance is the level of light at a point, so zero is black and in an 8 bit image white would be 255. Color is not effected because Luminance contains no color. on the other hand when you work in RGB each color point has a hue, and luminance attached to it. So black is 0,0,0 and white is 255, 255, 255. As you adjust the values of the R,G,B channels you are effecting not only the lightness but the color value of that point. So that is why Photoshop professionals work in LAB space because it separates light levels from color, the same is in the LCH tool in CNX2. The LCH tool also lets you adjust Chroma (saturation of a particular color, Color lightness (or darkness) and Hue (you can change one color to another ( very hard to do in RGB without artifacts but very easy in LAB space and LCH.