How multiple colours are emitted in a single led?

Two LED chips are usually connected parallel & in reversed direction to eachother in one single packet.

Consider the image :

One color is obtained when current is flowing in one direction from LED & other color when other LED is biased from other direction.
Both LEDs cannot emit light at same time.

divya nairsay yellow & violet that Iam working on?

From what I know, bicolored LEDs can emit only Red,Green & Blue, but not yellow,violet,Indigo & Orange.
Edit : Different colors are possible if different material is used.

multicolor leds hav separate R,G,B dies in a single package with common cathode..
U can see those if its clear glass led...RGB colors can be mixed via pwm to generate other colors...
if ur talking abt leds available in different colors..
Wavelength of the light emitted by led depends on the band gap energy of the materials used for pn junctions ..color perceived depends on wavelength..
so different materials gives diff colors...
Indium gallium nitride is for violet..

For More Information :
I just read a blog on multi-colored LEDs, there's one more method which can emit shades of color.

In this case, there is two input & a single output.
When any one LED is given i/p current the output is obtained, but when both LEDs are given i/p current, it is possible to get four shades of color when different ratio of forward current is passed through each of the i/p.
Totally 4 colors are obtained by this method using two LEDs.

If the voltage applied to the LED is lesser then the colour output got from it will be a lesser shade of the actual color it has to emit

That way light color of several known intensity can be generated.

thanx #-Link-Snipped-# sir,@ conqueror sir,&@ cyborg 009.The ckt. diagram cleared the confusion.

In PWM method,leds are time multiplexed to give desired color..
In this way brightness of resultant color can be maintained fairly uniform..
Tred =Tgreen=Tblue=10ms >> white color
Tred =Tblue=10ms >> violet color
with 8bit PWM ,256 colors can be obtained...
see this

PWM concept is new to me.Great to know about this concepts.😎