How to find the best way to fade in/out?
Assuming you have the control side of things sorted to adequately vary the lights across their full spectrum I would suggest a proper light intensity sensor such as the BH1750.  Sensors such as this are only a few dollars, have digital interfaces and can measure the true lux from 0 to 65535.  LDRs might get you a ballpark figure but require proper calibration to obtain light intensity readings.

Bare in mind that the intensity of particular wavelengths of light as perceived by the human eye will differ to that of measured intensities so the eventual curve you decide to use to control these LEDs will depend on the effect you're after.