How to calculate illuminance from the output of a TCS34725 color sensor?
In the chip's datasheet on page 8 in the chapter "RGBC Operation" (emphasis by me):


  The RGBC engine contains RGBC gain control (AGAIN) and four
  integrating analog-to-digital converters (ADC)for the RGBC
  photodiodes. The RGBC integration time (ATIME) impacts both the
  resolution and the sensitivityof  the  RGBC  reading.  Integration  of
  all  four  channels  occurs  simultaneously  and  upon  completion  of
  theconversion cycle, the results are transferred to the color data
  registers. This data is also referred to as channelcount.


So, if you read the values from the data registers, you get simple count values, which were determined by the ADCs connected to the channels. To get a value in lux, you need to calculate that yourself.

Adafruit has a library for their breakout board with this chip. In their tutorial it seems, that the library outputs a value in lux. Maybe you can also use this library, or you can check, how they are converting the values from the sensor to lux values.