RGB is color mode which is suitable for screen, CMYK for printing with process colors. In most cases to print image you will first work with it in RGB color mode because you are watching your work on screen then you will need to convert to CMYK. RGB model is created from 3 main colors Red, Green and Blue, CMYK color mode is created from 3 opposite colors to Red, Green and Blue and they are Cyan, Magenta and Yellow plus Black color Ink. You can manage colors in RGB mode by increasing or decreasing participation of Cyan, Magenta or Yellow which are opposite colors on color wheel. In Curves dialog you can increase participation of Red color for example but also decrease by increasing participation of Cyan (pull curve in opposite side).