The Color Plug is a small board containing the TAOS TCS414 digital light color sensor.
This chip measures the color chromaticity and illuminance of ambient light with up to 16-bits of resolution. An array of 16 tiny photodiodes have red, green, blue and neutral filters, their transfer functions factory trimmed to give very precise repeatability.
An I2c bus interface running at 3.3v (address 0x39) transfers captured data at up to 400kHz. All the power and signal lines are brought out to both sides of the board for daisy-chaining other i2c-conformant boards.
The Assembled Color Sensor Plug comes with the SMD-sized TCS414 chip and decoupling capacitor already soldered onto the board. Headers for this and all other JeeLabs plugs can be found here (straight) and here (right angle).
The PCB Only version contains just the printed circuit board for the Color Sensor Plug. No parts, no headers.
The documentation and full design details are freely available.