In the kit driving a Bicolor LED, Bi-color LEDs often contain both green and red LEDs connected in ‘inverse parallel’. This means if current flows one way through the device the LED lights green, and if current flows the other way the LED lights red.
The kit driving a bicolor LED is similar to some flasher circuits using IC555 as an Astable multivibrator. The only difference lies in the way it flash using bi-color LEDs. When output at pin 3 of IC 555 goes high it operates on group of LEDs. By inverting the IC’s low output by PNP transistor BC557 the other group of LEDs is made to flash. This circuit alternately flashes a red/green bi-coloured LED. When the output of pin 3 of 555 timer IC is high then the potential difference across the NPN transistor BC557 and pin 3 will same so there will be no current flow across the transistor so the green LED will be turn off.