I've also seen it done the other way, using red, green, and blue LED's reflected off the paper onto a single LDR. There are issues such as the exact color of each LED not being perfect R/G/B, the light intensities of each not being the same, and the exact angle/distance from each LED to the sensor not being identical, but depending on your application it could be more than good enough.
Accuracy could of course be improved by more careful construction, adjustment of each LED's drive current to better equalize the intensity, and some calibration in software.