Adafruit RGB sensor schematic
The first part is simply a voltage regulator that creates a 3.3V supply from whatever Vin is, presumably 5V.



The third part...



Is a logic level translator that converts the 3.3V signals SCL_3V and SDA_3V into logic levels at Vin level.

Whenever either of the 3V signals on the left are high the associated MOSFET will be off and the signal on the right is pulled to Vin by the pull-up. 

When the signal on the left is low, the MOSFET will be turned on because the   gate voltage is tied to 3.3V. This will in turn pull the output line low.

The pull-ups on the left ensure the signals on the left are high when whatever is driving those lines.. well.. isn't.